Protein electron mediator

ABSTRACT

The problem to be resolved is to provide an electron mediator and a fusion body with high affinity with an enzyme, a measuring method using extracellular secretion type cytochrome and an enzyme, an electrode, and a sensor. 
     The present invention relates to an electron mediator for glucose oxidoreductase comprising extracellular secretion type cytochrome, a fusion body in which the electron mediator is fused with glucose oxidoreductase, a composition for glucose measurement including the electron mediator or fusion body, a gene encoding a new extracellular secretion type cytochrome, and a measurement method using extracellular secretion type cytochrome and an enzyme, an electrode, and a sensor.

This application is a continuation of U.S. application Ser. No. 12/857,880, filed Aug. 17, 2010 now U.S. Pat. No. 8,716,442, which is a continuation of International Application No. PCT/JP2010/057699, filed Apr. 30, 2010.

FIELD OF THE INVENTION

The present invention relates to an electron mediator for glucose oxidoreductase used for enzyme sensors and bio-batteries, etc.

BACKGROUND OF THE INVENTION

An electron mediator is also referred to as an electron transfer substance, meaning a substance that has a function to receive an electron from an electron donor and/or a function to give an electron to an electron acceptor, and it is known that such mediators exist in an oxidized form or a reduced form. The electron mediator is used for colorimetric determination of enzyme activity, etc. and plays a role in giving an electron received from an enzyme through an enzyme electrode.

An enzyme electrode is used for an enzyme sensor, etc. to measure the content of a specific substance (target substance) included in a biological sample by utilizing an enzyme. Various enzyme sensors have already been commercialized, but for example, a glucose sensor, etc. used to measure glucose concentration in the blood is known, and for an electrode, an enzyme electrode in which an enzyme is immobilized on the surface of an electrode such a gold electrode, a platinum electrode, or a carbon electrode is used. In a glucose sensor, a substance generated by a reaction between glucose and an enzyme in a sample is electrochemically detected and quantitated.

As an example of use of an enzyme electrode, a bio-battery utilizing an electron generated by enzyme treatment with glucose or ethanol as a substrate is known. As examples of bio-batteries, self-contained batteries and environment-friendly batteries, etc. have been focused on in recent years.

In general, because an enzyme is not prone to direct oxidation or reduction on an electrode surface, an electron mediator that plays a role in receiving an electron from the enzyme and giving it to the electrode is necessary in order to measure the glucose concentration in a biological sample.

As an electron mediator, a protein electron mediator has been developed, and as an electrode using a protein electron mediator, an enzyme electrode using cytochrome C, cytochrome b562, and cytochrome c551, etc. (Patent Document 1) is known. Furthermore, a fusion protein (Patent Document 2), etc. in which the cytochrome C domain of quinohemoprotein ethanol dehydrogenase derived from Comamonas testosterone, which is a protein electron mediator, is fused with pyrroloquinoline quinone glucose dehydrogenase has been developed. At the same time, glucose sensors using flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase derived from Burkholderia cepacia have been developed (Patent Documents 3 and 4) and an enzyme electrode including an enzyme derived from Burkholderia cepacia and a production method thereof have also been reported (Patent Document 5).

The cytochrome C and cytochrome b562 described in Patent Document 1 are both electron transfer proteins present in a cell such as a cell membrane or periplasm. When this electron transfer protein performs electron reception to an electrode in the absence of other electron mediators, it requires an amount 100 times as much at a molar ratio compared to glucose dehydrogenase, and it can therefore be said that it has very low affinity with glucose dehydrogenase and glucose oxidase. The fusion protein described in Patent Document 2 requires a massive amount of as many as 1,000 units when glucose measurement is performed. In addition, in glucose measurements performed using the fusion protein, the range of measurable glucose concentrations is 5 mM or less, but it can be said that this is very narrow compared to the fact that the upper limit of glucose concentration generally obtained in blood sugar measurements is 20 to 40 mM. The sensors described in Patent Documents 3, 4, and 5 use an enzyme comprising three subunits including an α subunit, in which the original wild-type enzyme is a catalytically active subunit, a β subunit, in which the original wild-type enzyme is an electron transfer subunit, and a γ subunit, and these are sensors in which a trimer of αβγ subunits or a dimmer of αβ subunits are immobilized. Because the enzyme originally has an electron transfer subunit (β subunit) equivalent to cytochrome C as a wild type, it is an enzyme capable of direct electron migration to an electrode. However, because the β subunit is a membrane-bound cytochrome and the enzyme is a membrane-bound enzyme derived from Burkholderia cepacia, complicated treatments such as solbilization treatment are required in order to obtain a subunit of the enzyme or the enzyme. Additionally, because those that have been subjected to solbilization treatment are unstable, it is difficult to maintain the enzyme or its subunit structure once processes such as desiccation is performed. In addition, according to a presentation at the Annual Meeting of the Society for Biotechnology, Japan (Oct. 28 to 30, 2002), the enzyme has poor substrate specificities to maltose activity and galactose activity when the substrate specificity to glucose activity is 100%, the substrate specificities being 40% and 105% for SM4 strains, 43% and 132% for JCM5506 strains, 57% and 123% for JCM550 strains, 83% and 108% for JCM2800 strains, 74% and 117% for JCM2801 strains, 38% and 104% for IFO14595 strains, and 74% and 148% for IFO15124 strains from among each strain of Burkholderia cepacia, and according to the presentation of the presenter, because it has high active properties on maltose and galactose, there are problems for using it for a self-blood glucose meter.

In Patent Documents 3, 4, and 5, a glucose sensor using a FAD-dependent enzyme and electrode are disclosed, but here, an enzyme is immobilized to a powder conductor or a carbon particle that are different from the electrode. Moreover, platinum, etc. is used for a counter electrode, which is a second electrode, and the second electrode does not have redox properties. Furthermore, no polymer molecules are used for supporting an enzyme against an electrode.

As other types of cytochrome, extracellular secretion type cytochrome b562 derived from Phanerochaete chrysosporium is known, and regarding recombinant cytochrome in which the cytochrome has been produced with yeast, it is known that it has sorbability to cellulose and chitin (Non-patent Document 1). However, regarding the cytochrome, its function as a mediator for glucose oxidoreductase was not known. Furthermore, because the recombinant cytochrome has yeast as a host, excessive sugar chains are added, causing a large molecular weight, and therefore, the solid content per mole is high, and for a reagent for a sensor in which it is necessary to dissolve a reagent with a small amount of blood, there has been a need for a recombinant protein with less glycosylation and a smaller molecular weight.

PRIOR ART DOCUMENTS Patent Documents

-   Non-patent Document 1: International Publication No. 02/073181     pamphlet -   Non-patent Document 2: International Publication No. 05/030807     pamphlet -   Non-patent Document 3: International Publication No. 05/023111     pamphlet -   Non-patent Document 4: International Publication No. 02/036779     pamphlet -   Non-patent Document 5: International Publication No. 09/037,838     pamphlet

Non-Patent Document

-   Non-patent Document 1: Makoto Yoshida, Kiyohiko Igarashi, et al.,     Applied and Environmental Microbiology, 4548-4555 (2000)

SUMMARY OF THE INVENTION Problem to be Resolved by the Invention

As described above, in the electron transfer proteins or fusion proteins described in Patent Documents 1, 2, 3, 4, and 5, a response current value that can be considered sufficient has not been obtained for practical use.

Therefore, an objective of the present invention is to solve the above problem and to provide a protein electron mediator with high affinity with an enzyme as well as a fusion body of an enzyme with high specificity to a substrate and the electron mediator, as well as a composition including the same, a method for measuring a substrate using the same, and a sensor for a substrate, as well as a bio-battery including the same, etc.

Means of Solving the Problem

As an alternative to electron mediators that have been used conventionally, the present inventor widely searched an electron mediator in the natural world in which a response current value considered practically sufficient could be obtained and consequently found that soluble extracellular secretion type cytochrome delivered excellent performance as an electron mediator of glucose oxidoreductase, and completed the present invention.

In other words, the present invention relates to the following embodiments.

[Embodiment 1]

An electron mediator for glucose oxidoreductase comprising extracellular secretion type cytochrome.

[Embodiment 2]

The electron mediator according to Embodiment 1, wherein glucose concentration measurement is possible with less than 100 times as many molar numbers of the electron mediator as the molar numbers of glucose oxidoreductase.

[Embodiment 3]

The electron mediator according to Embodiment 1, wherein the extracellular secretion type cytochrome is derived from a bacterium belonging to filamentous bacteria.

[Embodiment 4]

The electron mediator according to Embodiment 3, wherein the filamentous bacterium is a bacterium of genus Aspergillus.

[Embodiment 5]

The electron mediator according to Embodiment 4, wherein the bacterium of genus Aspergillus is Aspergillus terreus or Aspergillus oryzae.

[Embodiment 6]

An electron mediator for glucose oxidoreductase comprising a polypeptide of the following (a), (b) or (c):

-   (a) a polypeptide comprising the amino acid sequence depicted in SEQ     ID NO: 2 or SEQ ID NO: 4; -   (b) a polypeptide comprising an amino acid sequence in which one or     several amino acids have been substituted, deleted, inserted or     added in the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO:4 and     having an electron mediator function; -   (c) a polypeptide comprising an amino acid sequence having 70% or     more homology with the amino acid sequence of SEQ ID NO: 2 or SEQ ID     NO: 4 and having an electron mediator function.     [Embodiment 7]

A polynucleotide of the following (a), (b), (c), (d), (e) or (f):

-   (a) a polynucleotide encoding a polypeptide comprising the amino     acid sequence depicted in SEQ ID NO:2 or SEQ ID NO: 4; -   (b) a polynucleotide encoding a polypeptide that comprises an amino     acid sequence in which one or several amino acids have been     substituted, deleted, inserted or added in the amino acid sequence     of SEQ ID NO: 2 or SEQ ID NO: 4 and has an electron mediator     function; -   (c) a polynucleotide encoding a polypeptide that comprises an amino     acid sequence having 70% or more homology with the amino acid     sequence of SEQ ID NO: 2 or SEQ ID NO: 4 and has an electron     mediator function; -   (d) a polynucleotide including the base sequence depicted in SEQ ID     NO: 1 or SEQ ID NO: 3 and encoding a polypeptide having an electron     mediator function; -   (e) a polynucleotide hybridizing under stringent conditions with a     polynucleotide comprising a base sequence complementary to the     polynucleotide comprising (d) and encoding a polypeptide having an     electron mediator function; -   (f) a polynucleotide including a base sequence having 70% or more     homology with a polynucleotide comprising (d) and encoding a     polypeptide having an electron mediator function.     [Embodiment 8]

A recombinant vector including the gene according to Embodiment 7.

[Embodiment 9]

A transformant transformed by the recombinant vector according to Embodiment 8.

[Embodiment 10]

The transformant according to Embodiment 9, wherein a host is a filamentous bacterium.

[Embodiment 11]

A method for producing extracellular secretion type cytochrome having an electron mediator function, wherein a transformant transformed by a recombinant vector including a gene encoding extracellular secretion type cytochrome is cultured in a nutrient medium to collect extracellular secretion type cytochrome.

[Embodiment 12]

Cytochrome obtained by the method of production according to Embodiment 11.

[Embodiment 13]

A fusion body in which extracellular secretion type cytochrome and glucose oxidoreductase are fused.

[Embodiment 14]

The fusion body according to Embodiment 13 with which glucose in a range greater than 5 mM can be measured in the absence of other electron mediators.

[Embodiment 15]

The fusion body according to Embodiment 13, wherein the glucose oxidoreductase is glucose dehydrogenase.

[Embodiment 16]

The fusion body according to Embodiment 13, wherein the glucose oxidoreductase is glucose oxidase.

[Embodiment 17]

A fusion body having both functions of an electron mediator function and a substrate-oxidizing function within a single molecule, and having (a) and (b) of the following (a), (b), and (c), or having (a) and (b) as well as having (c) between (a) and (b).

-   (a) an amino acid sequence of extracellular secretion type     cytochrome; -   (b) an amino acid sequence of glucose oxidoreductase; -   (c) a linker sequence binding the amino acid sequence of (a) with     the amino acid sequence of (b).     [Embodiment 18]

The fusion body according to Embodiment 17, wherein the linker sequence is GDCSGDGGGGSGPEPVPVPDG.

[Embodiment 19]

A polypeptide of the following (a), (b) or (c):

-   (a) a polypeptide comprising the amino acid sequence depicted in SEQ     ID NO: 23, SEQ ID NO: 25, SEQ ID NO: 27 or SEQ ID NO: 29; -   (b) a polypeptide comprising an amino acid sequence in which one or     several amino acids have been substituted, deleted, inserted or     added in SEQ ID NO: 23, SEQ ID NO: 25, SEQ ID NO: 27 or SEQ ID NO:     29 and having an electron mediator function and a glucose-oxidizing     function; -   (c) a polypeptide comprising an amino acid sequence having 70% or     more homology with the amino acid sequence of SEQ ID NO: 23, SEQ ID     NO: 25, SEQ ID NO: 27 or SEQ ID NO: 29 and having an electron     mediator function and a glucose-oxidizing function.     [Embodiment 20]

A gene encoding the fusion body according to Embodiment 13.

[Embodiment 21]

A recombinant vector including the gene according to Embodiment 20.

[Embodiment 22]

A transformant transformed by the recombinant vector according to Embodiment 21.

[Embodiment 23]

A method for producing a fusion body according to Embodiment 13, wherein the transformant according to Embodiment 22 is cultured in a nutrient medium to collect a fusion body in which extracellular secretion type cytochrome and glucose oxidoreductase are fused.

[Embodiment 24]

A composition for glucose measurement including either the electron mediator according to Embodiment 1 and glucose oxidoreductase, or the fusion body according to Embodiment 13.

[Embodiment 25]

A bio-battery including either the electron mediator according to Embodiment 1 and glucose oxidoreductase, or the fusion body according to Embodiment 13.

[Embodiment 26]

A method for measuring an enzyme activity using the electron mediator according to any of Embodiments 1 to 6, or an electron mediator for glucose oxidoreductase comprising the extracellular secretion type cytochrome according to Embodiment 12.

[Embodiment 27]

A measuring method for measuring the activity of an enzyme using extracellular secretion type cytochrome, a substrate, and an enzyme, comprising:

-   I) a step of oxidizing the substrate with the enzyme; -   II) a step of accepting an electron generated within the enzyme in     step I with the extracellular secretion type cytochrome; -   III) a step of detecting changes in the extracellular secretion type     cytochrome generated by step II; and -   IV) a step of associating the quantity of changes per unit time in     the extracellular secretion type cytochrome detected in step III     with the activity of the enzyme.     [Embodiment 28]

A measuring method for measuring an activity of an enzyme using extracellular secretion type cytochrome, a substrate, an enzyme, and an electron acceptor, comprising:

-   I) a step of oxidizing the substrate with the enzyme; -   II) a step of accepting an electron generated within the enzyme in     step I with the extracellular secretion type cytochrome; -   III) a step of accepting an electron in the extracellular secretion     type cytochrome generated by step II with the electron acceptor; -   IV) a step of detecting changes in the electron acceptor generated     by step III; and -   V) a step of associating the quantity of changes per unit time in     the electron acceptor detected in step 1V with the activity of the     enzyme.     [Embodiment 29]

A method for measuring a subject to be measured using extracellular secretion type cytochrome, an enzyme, and an electron acceptor, comprising:

-   A) a step of oxidizing the subject to be measured with the enzyme; -   B) a step of accepting an electron generated within the enzyme in     step A with the extracellular secretion type cytochrome; -   C) a step of accepting an electron in the extracellular secretion     type cytochrome generated by step B with the electron acceptor; -   D) a step of detecting changes in the electron acceptor generated by     step C; and -   E) a step of associating the quantity of changes in the electron     acceptor detected in step D with the amount or concentration of the     subject to be measured.     [Embodiment 30]

The measuring method according to Embodiment 29, wherein the amount of the extracellular secretion type cytochrome is smaller than 100 times that of the enzyme used in measurement.

[Embodiment 31]

The measuring method according to Embodiment 29, wherein the extracellular secretion type cytochrome is derived from a bacterium belonging to filamentous bacteria.

[Embodiment 32]

The measuring method according to Embodiment 31, wherein the filamentous bacterium is a bacterium of genus Aspergillus.

[Embodiment 33]

The measuring method according to Embodiment 32, wherein the bacterium of genus Aspergillus is Aspergillus terreus or Aspergillus oryzae.

[Embodiment 34]

The measuring method according to Embodiment 29, wherein the extracellular secretion type cytochrome is the electron mediator according to the Embodiment 6.

[Embodiment 35]

The measuring method according to Embodiment 29, wherein the enzyme is oxidoreductase or dehydrogenase.

[Embodiment 36]

The measuring method according to Embodiment 29, wherein the enzyme is an enzyme acting on glucose.

[Embodiment 37]

The measuring method according to Embodiment 29, wherein the enzyme is flavin adenine dinucleotide-dependent.

[Embodiment 38]

The measuring method according to Embodiment 29, wherein the enzyme is flavin adenine dinucleotide-dependent glucose dehydrogenase.

[Embodiment 39]

The measuring method according to Embodiment 29, wherein the subject to be measured is glucose.

[Embodiment 40]

The measuring method according to Embodiment 29, wherein the electron acceptor is an electrode.

[Embodiment 41]

The measuring method according to Embodiment 29, wherein the electron acceptor is a redox compound.

[Embodiment 42]

The measuring method according to Embodiment 29, wherein the detection in step D is performed with a current, an energization charge amount or a spectrographic amount.

[Embodiment 43]

The measuring method according to Embodiment 29, wherein the changes detected in step D are obtained via a reaction of one or several redox substances.

[Embodiment 44]

The measuring method according to Embodiment 29, wherein a measurable concentration of the subject to be measured is greater than 5 mM.

[Embodiment 45]

The measuring method according to Embodiment 29, wherein the extracellular secretion type cytochrome and the enzyme are fused.

[Embodiment 46]

The measuring method according to Embodiment 45, wherein the enzyme is glucose dehydrogenase or glucose oxidase.

[Embodiment 47]

The measuring method according to Embodiment 45, wherein the fusion body of the extracellular secretion type cytochrome and the enzyme is the fusion body according to Embodiment 17.

[Embodiment 48]

The measuring method according to Embodiment 45, wherein the fusion body of the extracellular secretion type cytochrome and the enzyme is the polypeptide according to Embodiment 19.

[Embodiment 49]

A method for measuring a subject to be measured using extracellular secretion type cytochrome, an enzyme, a first electrode, and a second electrode, comprising:

-   F) a step of oxidizing the subject to be measured with the enzyme; -   G) a step of accepting an electron generated within the enzyme in     step F with the extracellular secretion type cytochrome supported by     the first electrode; -   H) a step of accepting an electron in the extracellular secretion     type cytochrome generated by step G with the first electrode; -   I) a step of detecting a current or an energization charge amount     flowing between the first electrode and the second electrode by step     H; and -   J) a step of associating the current or the energization charge     amount detected in step I with the amount or concentration of the     subject to be measured.     [Embodiment 50]

The measuring method according to Embodiment 49, wherein the enzyme is supported by the first electrode.

[Embodiment 51]

The measuring method according to Embodiment 49, wherein the enzyme and the extracellular secretion type cytochrome are fused.

[Embodiment 52]

The measuring method according to Embodiment 49, wherein the enzyme and the extracellular secretion type cytochrome or a fusion body thereof are supported by the first electrode through a polymer molecule.

[Embodiment 53]

The measuring method according to Embodiment 49, wherein step H is induced by the application of a voltage to the first electrode.

[Embodiment 54]

The measuring method according to Embodiment 53, wherein the application of a voltage to the first electrode is performed on the second electrode.

[Embodiment 55]

The measuring method according to Embodiment 54, wherein the second electrode oxidizes and reduces.

[Embodiment 56]

The measuring method according to Embodiment 55, wherein the second electrode is Ag/AgCl.

[Embodiment 57]

The measuring method according to Embodiment 53, wherein using an oxidizing and reducing third electrode, the application of a voltage to the first electrode is performed on the third electrode.

[Embodiment 58]

The measuring method according to Embodiment 57, wherein the third electrode is Ag/AgCl.

[Embodiment 59]

The measuring method according to Embodiment 49, wherein step F and step G are performed concurrently.

[Embodiment 60]

The measuring method according to Embodiment 49, wherein the application of a voltage to the first electrode is performed prior to step F.

[Embodiment 61]

A method for measuring a subject to be measured using extracellular secretion type cytochrome and an enzyme, comprising:

-   K) a step of oxidizing the subject to be measured with the enzyme; -   L) a step of accepting an electron generated within the enzyme in     step K with the extracellular secretion type cytochrome; -   M) a step of detecting changes in spectroscopic characteristics of     the extracellular secretion type cytochrome generated by step L; and -   N) a step of associating the characteristic changes detected in step     M with the amount or concentration of the subject to be measured.     [Embodiment 62]

An electrode for measuring the concentration or amount of a subject to be measured wherein extracellular secretion type cytochrome and an enzyme are supported.

[Embodiment 63]

The electrode according to Embodiment 62, wherein the extracellular secretion type cytochrome and the enzyme are supported by a polymer molecule.

[Embodiment 64]

The electrode according to Embodiment 62, wherein extracellular secretion type cytochrome and the enzyme are fused.

[Embodiment 65]

The electrode according to Embodiment 62, wherein the electrode includes any of carbon, gold, platinum or palladium.

[Embodiment 66]

A sensor for performing measurement of a subject to be measured included in a sample solution, comprising at least:

-   i) an insulating first substrate; -   ii) first and second electrodes placed on the first substrate; -   iii) a reagent layer placed on the first electrode; and -   iv) a sample solution-holding part contacting the first electrode or     the reagent layer and the second electrode, wherein the reagent     layer includes extracellular secretion type cytochrome and an enzyme     is placed on either the reagent layer or the sample solution-holding     part.     [Embodiment 67]

The sensor according to Embodiment 66, wherein either of the first and second electrodes includes any of carbon, gold, platinum or palladium.

[Embodiment 68]

The sensor according to Embodiment 66, wherein the second electrode is an oxidizing and reducing electrode.

[Embodiment 69]

The sensor according to Embodiment 68, wherein the second electrode is Ag/AgCl.

[Embodiment 70]

The sensor according to Embodiment 66, wherein the reagent layer is placed only on the first electrode.

[Embodiment 71]

The sensor according to Embodiment 66, wherein the extracellular secretion type cytochrome and the enzyme are fused.

[Embodiment 72]

The sensor according to Embodiment 66, wherein the amount of the extracellular secretion type cytochrome is smaller than 100 times that of the enzyme used in measurement.

[Embodiment 73]

The sensor according to Embodiment 66, wherein the extracellular secretion type cytochrome is derived from any of a bacterium belonging to filamentous bacteria, a bacterium belonging to genus Aspergillus, Aspergillus terreus or Aspergillus oryzae.

[Embodiment 74]

The sensor according to Embodiment 66, wherein the extracellular secretion type cytochrome is the electron mediator according to the Embodiment 6.

[Embodiment 75]

The sensor according to Embodiment 71, wherein the fusion body of the extracellular secretion type cytochrome and the enzyme is the fusion body according to Embodiment 17.

[Embodiment 76]

The sensor according to Embodiment 71, wherein the fusion body of the extracellular secretion type cytochrome and the enzyme is the polypeptide according to Embodiment 19.

[Embodiment 77]

The sensor according to Embodiment 66, wherein the enzyme is oxidoreductase or dehydrogenase.

[Embodiment 78]

The sensor according to Embodiment 66, wherein the enzyme is an enzyme acting on glucose.

[Embodiment 79]

The sensor according to Embodiment 66, wherein the enzyme is flavin adenine dinucleotide-dependent.

[Embodiment 80]

The sensor according to Embodiment 66, wherein the enzyme is flavin adenine dinucleotide-dependent glucose dehydrogenase.

[Embodiment 81]

The sensor according to Embodiment 66, wherein the subject to be measured is glucose.

[Embodiment 82]

The sensor according to Embodiment 66 that further includes a second substrate having a v) notch part, wherein the second substrate forms at least a portion of the sample solution-holding part.

[Embodiment 83]

The sensor according to Embodiment 82, wherein the shape of the notch part is rectangular or U-shaped.

[Embodiment 84]

The sensor according to Embodiment 82 that further includes a vi) third substrate, wherein the third substrate forms at least a portion of the sample solution-holding part.

[Embodiment 85]

The sensor according to Embodiment 66, wherein a measurable concentration of the subject to be measured is greater than 5 mM.

Effects of the Invention

Using the soluble extracellular secretion type cytochrome of the present invention for glucose oxidoreductase allows for obtaining an electron mediator with high affinity with an enzyme as well as for direct electron transfer to an electrode. Furthermore, extracellular secretion type cytochrome has excellent properties that membrane-bound cytochrome, which is known as a conventional electron mediator for glucose oxidoreductase, does not have, such as the fact that it is easy to collect because it is a soluble protein and that it has high stability because it is not insolubilized during purification. Additionally, because extracellular secretion type cytochrome has high affinity with glucose oxidoreductase, it became possible to reduce the amount of use of the cytochrome when it is used for glucose measurement as an electron mediator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the results of a measurement of response current values to glucose based on an enzyme electrode using the extracellular secretion type cytochrome of the present invention.

FIG. 2 shows graphs illustrating the results of a measurement of response current values to glucose based on an enzyme electrode using the extracellular secretion type cytochrome of the present invention.

FIG. 3 is a graph illustrating the results of a measurement of electron-donating ability to cytochrome C with regard to a fusion body of the present invention. The upper curve indicates AoCytb-AtGLD and the lower curve indicates AtGLD.

FIG. 4 is an exploded perspective diagram illustrating a sensor that is one embodiment of the present invention.

FIG. 5 is a perspective diagram illustrating the skeleton framework of a measurement system.

FIG. 6 is a block diagram illustrating the skeleton framework of the measurement system.

FIG. 7 is a graph illustrating the relationship between glucose concentration and response current when a sensor that is one embodiment of the present invention is used.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is characterized by the use of extracellular secretion type cytochrome as an electron mediator for an enzyme such as glucose oxidoreductase.

In this research, the present inventors found that extracellular secretion type cytochrome delivered excellent performance as an electron mediator of glucose oxidoreductase. The extracellular secretion type cytochrome of the present invention is a natural protein substance produced by living organisms, and therefore, it also provides excellent safety.

The glucose oxidoreductase related to the present invention is an enzyme that catalyzes a redox reaction between glucose and an electron acceptor. The glucose oxidoreductase may be glucose oxidase or glucose dehydrogenase and is preferably glucose oxidoreductase in which the respective active properties on maltose and galactose are at most 10% when the active property on glucose is 100%, or more preferably glucose oxidoreductase in which the respective active properties on maltose and galactose are at most 8% when the active property on glucose is 100%, or still more preferably glucose oxidoreductase in which the respective active properties are at most 5%. Examples of the glucose oxidoreductase that may be used are glucose oxidase in which flavin adenine dinucleotide is a coenzyme, glucose dehydrogenase in which nicotinamide adenine dinucleotide is a coenzyme, or (flavin adenine dinucleotide-dependent) glucose dehydrogenase in which flavin adenine dinucleotide is a coenzyme.

In the case of glucose dehydrogenase in which flavin adenine dinucleotide is a coenzyme, if the respective active properties on maltose and galactose are 10% or less when the active property on glucose is 100%, there are no particular limitations regarding the type and origin, but glucose dehydrogenase derived from bacteria belonging to filamentous bacteria such as genus Aspergillus, genus Penicillium or genus Ganoderma as described in each pamphlet of International Publication No. 2004/058958, International Publication No. 2006/101239, and International Publication No. 2008/001903, or glucose dehydrogenase derived from genus Drosophila as described in Proc. Natl. Acad. Sci. USA Vol. 80, pp. 6286-6288 (1983) are preferred, and more specifically, glucose dehydrogenase derived from various strains (except for RIB40) of an Aspergillus terreus FERM BP-08578 strain as described in International Publication No. 2004/058958 pamphlet or an Aspergillus oryzae strain as described in International Publication No. 2008/001903 pamphlet are preferred. All types of such glucose dehydrogenase in which flavin adenine dinucleotide derived from bacteria belonging to filamentous bacteria is a coenzyme have a physicochemical property of catalyzing a reaction for oxidizing hydroxyl at position 1 of glucose and properties such as low active properties on maltose and galactose, has and all types have similar enzyme activity.

Furthermore, as glucose dehydrogenase in which flavin adenine dinucleotide is a coenzyme, for example, various types of modified glucose dehydrogenase may be used, wherein, through genetic engineering and modification of part of an amino acid sequence, the xylose active property/glucose active property on wild-type glucose dehydrogenase have been reduced and the active properties at a low-temperature range have been enhanced or various properties such as stability have been improved.

Examples of the above modification of part of an amino acid sequence can include an amino acid substitution selected from a group composed of D72A, G73D, G73A, G73S, G73C, G73Q, G73W, G73Y, G73E, G73H, R102H, Y228H, V356A, and P527L in an amino acid sequence of glucose dehydrogenase derived from an Aspergillus terreus FERM BP-08578 strain.

The glucose oxidoreductase used in the present invention can be readily prepared and acquired by any method known in the art based on, for example, the disclosure of each patent document referred above. Furthermore, the glucose oxidoreductase of the present invention includes those that include a signal sequence or a portion thereof in the N-terminal and those that have a signal sequence broken from the N-terminal.

The extracellular secretion type cytochrome of the present invention is one type of a hemoprotein containing heme iron. Extracellular secretion type cytochrome refers to a cytochrome extracellularly secreted and produced as a soluble protein by a wild strain, and specifically, it is a cytochrome having a signal sequence at the N-terminal of an amino acid sequence constituting the cytochrome. The cytochrome is not particularly limited as long as it is a cytochrome that is extracellularly secreted, but cytochrome b is preferred and cytochrome b562 is particularly preferred. Proteins produced by a gram-negative bacterium in which a gene encoding the cytochrome has been incorporated and intracellularly produced by a recombinant in which, among the genes encoding the cytochrome, a gene with the signal sequence part removed has been incorporated are also included. Furthermore, the extracellular secretion type cytochrome of the present invention includes those that include a signal sequence or a portion thereof in the N-terminal and those that have a signal sequence broken from the N-terminal.

An example of extracellular secretion type cytochrome useful as the above electron mediator can include extracellular secretion type cytochrome derived from a bacterium belonging to filamentous bacteria. There are no particular limitations regarding the origin as long as it is extracellular secretion type cytochrome, but extracellular secretion type cytochrome derived from a bacterium of genus Phanerochaete, genus Magnaporthe or genus Gibberella, which are white-rot fungi, as disclosed in Table 3 of Non-patent Document 1 may be used. Furthermore, extracellular secretion type cytochrome (comprising a polypeptide of Accession no. BAD95668.1, XP_(—)382527 or XP_(—)369170) encoded by a polynucleotide of Accessionno.AB 193288, XM_(—)382527 or XM_(—)369170, which are extracellular cytochrome genes described in Table 3 of Non-patent Document 1, or an electron transfer heme containing a domain positioned at the N-terminal of CDH (cellobiose dehydrogenase) as described in Table 3 of Non-patent Document 1 may be used. The extracellular secretion type cytochrome is preferably extracellular secretion type cytochrome derived from a bacterium belonging to fungi, or more preferably extracellular secretion type cytochrome derived from a bacterium of genus Aspergillus, or particularly preferably extracellular secretion type cytochrome derived from Aspergillus terreus or Aspergillus oryzae.

Furthermore, polypeptides constituting an electron mediator for glucose oxidoreductase of the present invention can include the following polypeptides:

-   (a) a polypeptide comprising the amino acid sequence depicted in SEQ     ID NO: 2 or SEQ ID NO: 4; -   (b) a polypeptide comprising an amino acid sequence in which one or     several amino acids have been substituted, deleted, inserted or     added in the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4 and     having an electron mediator function; or -   (c) a polypeptide comprising an amino acid sequence having homology     of 70% or more, preferably 75% or more, more preferably 80% or more,     still preferably 90% or more, or particularly preferably 95% or more     with the amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4 and     having an electron mediator function. Here, polypeptides comprising     the amino acid sequences depicted in SEQ ID NO: 2 and SEQ ID NO: 4     are extracellular secretion type cytochrome derived from strains of     Aspergillus terreus and Aspergillus oryzae, respectively.

Polypeptides of the present invention can include the following polynucleotides:

-   (a) a polynucleotide encoding a polypeptide comprising the amino     acid sequence depicted in SEQ ID NO: 2 or SEQ ID NO: 4; -   (b) a polynucleotide encoding a polypeptide that comprises an amino     acid sequence in which one or several amino acids have been     substituted, deleted, inserted or added in the amino acid sequence     of SEQ ID NO: 2 or SEQ ID NO: 4 and has an electron mediator     function; -   (c) a polynucleotide encoding a polypeptide that comprises an amino     acid sequence having homology of 70% or more with the amino acid     sequence of SEQ ID NO: 2 or SEQ ID NO: 4 and has an electron     mediator function; -   (d) a polynucleotide including the base sequence depicted in SEQ ID     NO: 1 or SEQ ID NO: 3 and encoding a polypeptide having an electron     mediator function; -   (e) a polynucleotide hybridizing under stringent conditions with a     polynucleotide comprising a base sequence complementary to the     polynucleotide comprising the base sequence (d) and encoding a     polypeptide having an electron mediator function; or -   (f) a polynucleotide including a base sequence having homology of     70% or more, preferably 75% or more, more preferably 80% or more,     still preferably 90% or more, or particularly preferably 95% or more     with a polynucleotide comprising the base sequence (d) and encoding     a polypeptide having an electron mediator function. Furthermore, a     polynucleotide of the present invention may be a sequence including     intron. In addition, a polynucleotide encoding the signal peptide     part of the above polypeptide may be a polynucleotide in which part     or the whole has been respectively substituted with an appropriate     part or removed by the host-vector system used.

In the present invention, an “electron mediator” refers to a substance having a function to receive an electron from an electron donor and/or a function to give an electron to an electron acceptor, and examples include a substance that receives an electron from glucose oxidoreductase and gives an electron to an electrode or other electron mediators. Therefore, a polypeptide having an “electron mediator” function substantially equivalent to extracellular secretion type cytochrome constituted with a protein comprising a polypeptide comprising the amino acid sequence depicted in SEQ ID NO: 2 or SEQ ID NO: 4 can also be used as an electron mediator for glucose oxidoreductase.

In the present invention, homology in an amino acid sequence or a base sequence refers to each sequence having a predetermined identity over the full length of a reference sequence to be compared. The uniformity percentage of such a sequence can be calculated using a public or commercially available software having an algorithm to compare a reference sequence as a query sequence. As an example, BLAST, FASTA or GENETYX (manufactured by Software Development Co., Ltd.), etc. may be used, and these may be used for default parameters.

In the present invention, an example of specific conditions for “hybridization under stringent conditions” in hybridization between polynucleotides includes incubation with 50% formamide, 5×SSC (150 mM of sodium chloride, 15 mM of trisodium citrate, 10 mM of sodium phosphate, 1 mM of ethylenediaminetetraacetate, pH7.2), 5×Denhardt's solution, 0.1% SDS, 10% dextran sulfate, and 100 μg/mL of altered salmon sperm DNA at 42° C. followed by washing the filter in 0.2×SSC at 42° C.

Furthermore, in the present invention, a “polynucleotide” refers to a molecule in which 100 or more phosphate esters (ATP (adenosine triphosphate), GTP (guanosine triphosphate), CTP (cytidine triphosphate), UTP (uridine triphosphate); or dATP (deoxyadenosine triphosphate), dGTP (deoxyguanosine triphosphate), dCTP (deoxycytidine triphosphate), dTTP (deoxythymidine triphosphate)) of a nucleoside in which purine or pyrimidine has been β-N-glycoside bound to a sugar are bound, and specifically, it includes chromosomal DNA encoding glucose oxidoreductase, mRNA transcribed from chromosomal DNA, cDNA synthesized from mRNA, and polynucleotides subjected to PCR amplification with those as templates. An “oligonucleotide” refers to a molecule in which 2 to 99 nucleotides are linked. A “polypeptide” refers to a molecule constituted with 30 or more amino acid residues bound to each other by amide binding (peptide binding) or a non-natural residue linkage, and furthermore, it also includes these with sugar chains added or those that have artificially undergone chemical modification, etc.

The extracellular secretion type cytochrome (polypeptide) of the present invention and a polynucleotide encoding the same can be readily prepared and acquired by any method known in the art. For example, extracellular secretion type cytochrome derived from a filamentous bacterium such as genus Aspergillus, genus Phanerochaete, genus Magnaporthe or genus Gibberella and a polynucleotide encoding the same can be prepared in accordance with the genetic engineering method described in the examples of the present specification. An intended extracellular secretion type cytochrome gene can be obtained by any method—including, for example, a PCR method using primer sets of SEQ ID NO: 5 and 6 or primer sets of SEQ ID NO: 7 and 8 with a cDNA library of Aspergillus terreus NIH2624 or Aspergillus oryzae RIB40 as a template, or a RT-PCR method with the whole RNA or mRNA extracted from Aspergillus terreus NIH2624 or Aspergillus oryzae RIB40 as a template—as a method of acquiring a polynucleotide. Furthermore, a polynucleotide encoding an intended extracellular secretion type cytochrome can be obtained by creating a pair of primer sets comprising a sense chain and an antisense chain from a known sequence of Accession no. AB193288, XM_(—)382527 or XM_(—)369170, which are extracellular cytochrome genes described in Table 3 of Non-patent Document 1, or a known sequence encoding an electron transfer heme containing a domain positioned at the N-terminal of CDH (cellobiose dehydrogenase) as described in Table 3 of Non-patent Document 1 followed by a method similar to the above methods. In addition, a polynucleotide encoding the next extracellular secretion type cytochrome can be acquired. In other words, a polypeptide constituting extracellular secretion type cytochrome includes a signal sequence at the N-terminal of an amino acid sequence, includes “Gly-Xaa-Met” (Xaa is any amino acid), which is a heme ligand, at any position between the 54th and the 110th positions from Met of the N-terminal, and includes Pro 9 amino acids away from Met of the sequence. Furthermore, it includes “Asn-Xaa-Thr” (Xaa is any amino acid) at any position between the 101st and the 160th positions from Met of the N-terminal and includes “Cys-Xaa-Xaa-Cys” (Xaa is any amino acid), which is an S—S binding domain, 7 amino acids away from Thr of the sequence. Additionally, it includes “His”, which is a heme ligand, at a position between the 158th and the 214th positions from Met of the N-terminal sequence. In a polynucleotide encoding the polypeptide, a polynucleotide encoding an intended extracellular secretion type cytochrome can be obtained by creating a pair of primer sets comprising a sense chain and an antisense chain from a known sequence followed by a method similar to the above method. Furthermore, if a primer is designed, the size of the primer (the number of bases) is 15 to 60 bases, desirably 20 to 50 bases, based on considerations of satisfactory specific annealing with template DNA. A complementary sequence between both primers is avoided so that a set or pair of primers (2 primers) comprising a sense chain (5′-terminal side) and an antisense chain (3′-terminal side) do not anneal with each other. Furthermore, in order to secure stable binding with the template DNA, it is desirable that the GC content is approximately 50% and GC-rich or AT-rich are not eccentrically located within the primers. Because the annealing temperature depends on the Tm value (melting temperature), primers approximated to each other at a Tm value of 55 to 65° C. are selected in order to obtain a PCR product with high specificity. In addition, it is necessary to note that the final concentration with use of the primers in a PCR is prepared to be approximately 0.1 to approximately 1 μM, etc. For primer design, commercially available software for designing a primer, such as, for example, Oligo™ [manufactured by National Bioscience Inc. (U.S.)] and GENETYX (manufactured by Software Development Co., Ltd.), can also be used.

Furthermore, the above oligonucleotide primer set can also be created by, for example, breaking the above cDNA with an appropriate restriction enzyme or can be synthesized in vitro by a known chemical synthesis technology as described in reference documents (for example, Carruthers (1982) Cold Spring Harbor Symp. Quant. Biol. 47:411-418; Adams (1983) J. Am. Chem. Soc. 105:661; Belousov (1997) Nucleic Acid Res. 25:3440-3444; Frenkel (1995) Free Radic. Biol. Med. 19:373-380; Blommers (1994) Biochemistry 33:7886-7896; Narang (1979) Meth. Enzymol. 68:90; Brown (1979) Meth. Enzymol. 68:109; Beaucage (1981) Tetra. Lett. 22:1859; U.S. Pat. No. 4,458,066).

A recombinant vector of the present invention is a cloning vector or an expression vector and a necessary vector is accordingly used depending on the type of polynucleotide used as an insert and the intended use, etc.

As for a transformed cell (transformant) of the present invention, for example, when extracellular secretion type cytochrome or a similar protein thereof is manufactured in volume, procaryotic cells such as Escherichia coli and Bacillus subtilis and eukaryotic cells such as yeast, fungi, filamentous bacteria, insect cells or mammal cells may be used, and they can be selected accordingly depending on the necessity or lack of necessity of a sugar chain or the necessity of other peptide modifications. The transformed cell is not particularly limited, but a filamentous bacterium is preferred, genus Aspergillus is further preferred, and Aspergillus oryzae is particularly preferred. These transformed cells can be prepared by introducing a recombinant vector into a cell by a known method such as electroporation, a calcium phosphate method, a liposome method, or a DEAE dextran method. Specific examples of a recombinant vector and a transformed cell include the recombinant vectors and recombinant fungi shown in the examples below.

Extracellular secretion type cytochrome can be acquired by a recombinant DNA technology using the above extracellular secretion type cytochrome polynucleotide. For example, extracellular secretion type cytochrome can be created in vitro by preparing RNA through in vitro transcription from a vector having the above polynucleotide and performing in vitro translation with this as a template. Furthermore, it can also be acquired similarly by DNA technology using a polynucleotide of Accession no. AB193288, XM_(—)382527 or XM_(—)369170, which are extracellular cytochrome genes described in Table 3 of Non-patent Document 1, or a polynucleotide encoding an electron transfer heme containing a domain positioned at the N-terminal of CDH (cellobiose dehydrogenase) as described in Table 3 of Non-patent Document 1. When extracellular secretion type cytochrome is produced through in vitro expression, extracellular secretion type cytochrome can be produced in vitro by creating a recombinant vector by inserting the above polynucleotide into a vector having a promoter to which RNA polymerase can bind and by adding this vector to the in vitro translation system of a rabbit reticulocyte lysate or a wheat germ extract, etc. including RNA polymerase corresponding to the promoter. Examples of promoters to which RNA polymerase can bind include T3, T7, and SP6. Examples of vectors including these promoters include pKA1, pCDM8, pT3/T718, pT7/319, and pBluescriptII.

When extracellular secretion type cytochrome is produced by expressing DNA in a microorganism such as Escherichia coli, extracellular secretion type cytochrome can be produced in volume within the microorganism by creating an expression vector in which the above polynucleotide has been recombined in an expression vector having an origin that can be replicated in a microorganism, a promoter, a ribosome binding site, a DNA cloning site, and a terminator sequence, etc. and by transforming the host cell with this expression vector followed by culturing the obtained transformant in an appropriate medium (for example, nutrient medium). At this time, if a start codon and a stop codon are added before and after any translation region and it is expressed, an extracellular secretion type cytochrome fragment including any region can also be obtained. Alternatively, it can be expressed as a fusion protein with another protein. Breaking this fusion protein with an appropriate protease allows for the acquisition of a target extracellular secretion type cytochrome. Examples of expression vectors for Escherichia coli include a pUC system, pBluescriptII, a pET expression system, a pGEX expression system or a pCold expression system.

When extracellular secretion type cytochrome is expressed with a eukaryotic cell and produced, extracellular secretion type cytochrome can be produced with a eukaryotic cell by creating a recombinant vector by inserting the above polynucleotide into an expression vector for a eukaryotic cell having a promoter, a splicing region, and a poly (A) addition site, etc., creating a transformant by introducing the vector into a eukaryotic cell, and culturing this transformant in an appropriate medium (for example, nutrient medium). Furthermore, using a polynucleotide of Accession no. AB193288, XM_(—)382527 or XM_(—)369170, which are extracellular cytochrome genes described in Table 3 of Non-patent Document 1, or a polynucleotide encoding an electron transfer heme containing a domain positioned at the N-terminal of CDH (cellobiose dehydrogenase) as described in Table 3 of Non-patent Document 1, extracellular secretion type cytochrome can be similarly produced with a eukaryotic cell. A recombinant vector can be maintained within a cell in a plasmid-like state or can also be incorporated into a chromosome and maintained. Examples of expression vectors include pKA1, pCDM8, pSVK3, pSVL, pBK-CMV, pBK-RSV, EBV vector, pRS, pYE82 or pUSA. If pIND/V5-His, pFLAG-CMV-2, pEGFP-N1 or pEGFP-C1, etc. is used as an expression vector, an extracellular secretion type cytochrome polypeptide can be also expressed as a fusion protein in which various tags such as a His tag, a FLAG tag or GFP, etc. have been added. As eukaryotic cells, mammalian cultured cells such as a monkey kidney cell COS-7 or a Chinese hamster ovary cell CHO, budding yeast, fission yeast, a fungus, a filamentous bacterium, a silkworm cell, and Xenopus oocyte, etc. are commonly used, and any eukaryotic cell may be used as long as it can express extracellular secretion type cytochrome, but it is particularly preferably a cell capable of producing heme b. In the case of a cell that does not have the ability to produce heme b, a gene necessary to produce heme b may be introduced into a cell in a manner similar to the case of producing the above extracellular secretion type cytochrome. In particular, a bacterium of genus Aspergillus is preferred and Aspergillus oryzae is most preferred. In order to introduce an expression vector into a eukaryotic cell, a known method such as electroporation, a calcium phosphate method, a liposome method, or a DEAE dextran method may be used.

In order to separate and purify an intended protein from a culture (a culture solution including a fungus body or an enzyme secreted outside a fungus body, and a medium composition, etc.) after extracellular secretion type cytochrome is expressed with a procaryotic cell or a eukaryotic cell, the process can be performed by combining known separation operations. For example, such operations may include treatment with a denaturation agent such as urea or a surfactant, heat treatment, pH treatment, ultrasonic treatment, enzyme digestion, a salting-out or solvent precipitation method, dialysis, centrifugation, ultrafiltration, gel filtration, SDS-PAGE, isoelectric focusing electrophoresis, ion-exchange chromatography, hydrophobic chromatography, reverse phase chromatography, and affinity chromatography (including a method utilizing a tag sequence and a method using a polyclonal antibody and a monoclonal antibody specific to UKC1), etc.

Extracellular secretion type cytochrome can be created by a known peptide synthesis method (Merrifield, R. B. J. Solid phase peptide synthesis I. The synthesis of tetrapeptide. J. Amer. Chem. Soc. 85, 2149-2154, 1963; Fmoc Solid Phase Peptide Synthesis. A Practical Approach. Chan, W. C. and White, P. D., Oxford University Press, 2000) based on, for example, the SEQ ID NO: 2 or SEQ ID NO: 4 or a similar sequence thereof. Furthermore, these peptides may be composed by a residue linkage other than natural amide binding. Examples of residue linkages other than natural amide binding include chemical binding or a coupling means with glutaraldehyde, N-hydroxysuccinimide ester, bifunctional maleimide, N,N′-dicyclohexylcarbodiimide (DCC) or N,N′-diisopropylcarbodiimide (DIC). Linking groups that may be used as alternatives to peptide binding include, for example, ketomethylene (e.g., —C(═O)—NH— for —C(═O)—CH₂—), aminomethylene (CH₂—NH), ethylene, olefin (CH═CH), ether (CH₂—O), thioether (CH₂—S), tetrazole (CN₄—), thiazole, retro-amide, thioamide, or ester (for examples, refer to Spatola (1983) in Chemistry and Biochemistry of Amino Acids, Peptides and Proteins, Vol. 7, pp 267-357, “Peptide Backbone Modifications,” Marcell Dekker, N.Y.).

Representative properties of the extracellular secretion type cytochrome of the present invention created by a method described above may include the following.

-   (1) Having a function to receive an electron from glucose     oxidoreductase and/or a function to give an electron to an electron     acceptor. -   (2) Molecular weight: Approximately 30 kDa (subunit molecular weight     when extracellular secretion type cytochrome recombined with a     filamentous bacterium into which the polynucleotide described in SEQ     ID NO: 1 or SEQ ID NO: 3 has been introduced is subjected to     polyacrylamide gel electrophoresis (SDS-PAGE)).

Furthermore, regarding the above molecular weight, because a sugar chain is originally added to the present enzyme, when the manner in which the sugar chain is attached changes according to the culture conditions or purification conditions, the molecular weight differs, and the sugar chain or an amino acid to be added also changes according to the type, etc. of a transformed cell or a vector system and the molecular weight differs. Further, if the amino acid sequence length or the manner in which the sugar chain is attached changes according to the type of polynucleotide to be introduced, the molecular weight differs.

-   (3) Presenting a red color. -   (4) Having characteristic absorption spectra at 427 nm, 531 nm, and     562 nm in a reduced form. -   (5) Being cytochrome b562. -   (6) Being a soluble protein. Furthermore, an electron mediator     comprising the extracellular secretion type cytochrome of the     present invention has high affinity with glucose oxidoreductase and     is an electron mediator that may be used at molar numbers preferably     less than 100 times, more preferably less than 50 times, still     preferably less than 20 times, or particularly preferably less than     10 times relative to the molar number of glucose oxidoreductase.

The present invention further provides a fusion body in which the above extracellular secretion type cytochrome and the above glucose oxidoreductase are artificially fused. The fusion body is a fusion body having both functions of an electron mediator function and a glucose-oxidizing function within a single molecule and may include—with reference to the following (a), (b), and (c)—a fusion body characterized in having (a) on the N-terminal side of an amino acid sequence and having (b) on the C-terminal side, a fusion body characterized in having (a) on the N-terminal side and having (b) on the C-terminal side as well as having (c) between (a) and (b), a fusion body characterized in having (a) on the C-terminal side of an amino acid sequence and having (b) on the N-terminal side, or a fusion body characterized in having (a) on the C-terminal side and having (b) on the N-terminal side as well as having (c) between (a) and (b).

-   (a) an amino acid sequence of extracellular secretion type     cytochrome; -   (b) an amino acid sequence of glucose oxidoreductase; and -   (c) a linker sequence binding the amino acid sequence of (a) to the     amino acid sequence of (b).

Furthermore, as long as the amino acid sequence of the extracellular secretion type cytochrome is an amino acid sequence constituting a polypeptide having the electron mediator function, it may be an amino acid sequence in which the amino acid sequence part on the C-terminal side of the polypeptide (for example; several to dozens of amino acids) is deleted accordingly when, for example, the polypeptide is on the N-terminal side of a fusion body polypeptide. Furthermore, when the amino acid sequence of the extracellular secretion type cytochrome is on the C-terminal side of the fusion body polypeptide, a sequence may be used in which the amino acid sequence part including part or the whole of the signal sequence is deleted accordingly from the entire amino acid sequence of the extracellular secretion type cytochrome. At the same time, as long as the amino acid sequence of the glucose oxidoreductase is an amino acid sequence constituting a polypeptide having the glucose-oxidizing function, it may be an amino acid sequence in which the amino acid sequence part on the N-terminal side of the polypeptide (for example, several to dozens of amino acids) is deleted accordingly. In particular, if the amino acid sequence of the glucose oxidoreductase is on the C-terminal side of the fusion body polypeptide, a sequence may be used in which the amino acid sequence part including part or the whole of the signal sequence is deleted accordingly from the entire amino acid sequence of the glucose oxidoreductase. The signal sequence part can be predicted using, for example, SignalP.

Polypeptides constituting a fusion body of the present invention can include the following polypeptides:

-   (a) a polypeptide comprising the amino acid sequence depicted in SEQ     ID NO: 23, SEQ ID NO: 25, SEQ ID NO: 27 or SEQ ID NO: 29; -   (b) a polypeptide comprising an amino acid sequence in which one or     several amino acids have been substituted, deleted, inserted or     added in the amino acid sequence depicted in SEQ ID NO: 23, SEQ ID     NO: 25, SEQ ID NO: 27 or SEQ ID NO: 29 and having an electron     mediator function; or -   (c) a polypeptide comprising an amino acid sequence having homology     of 70% or more, preferably 75% or more, more preferably 80% or more,     still preferably 90% or more, or particularly preferably 95% or more     with a polypeptide comprising the amino acid sequence depicted in     SEQ ID NO: 23, SEQ ID NO: 25, SEQ ID NO: 27 or SEQ ID NO: 29 and     having an electron mediator function. Here, the polypeptide     comprising the amino acid sequence depicted in SEQ ID NO: 23, SEQ ID     NO: 25, SEQ ID NO: 27 or SEQ ID NO: 29 is a polypeptide in which     extracellular secretion type cytochrome and glucose dehydrogenase     are fused and the polypeptide comprising the amino acid sequence     depicted in SEQ ID NO: 25 and SEQ ID NO: 29 includes a peptide     linker between the extracellular secretion type cytochrome and the     glucose dehydrogenase.

A gene having a base sequence encoding the above extracellular secretion type cytochrome and a gene having a base sequence encoding the above glucose oxidoreductase are linked by a common gene manipulation method and are turned into a single gene in order to construct a fusion body gene encoding a fusion body having the electron mediator function that the extracellular secretion type cytochrome has and the glucose-oxidizing function that the glucose oxidoreductase has within the same molecule.

As a gene having a base sequence encoding extracellular secretion type cytochrome, for example, the polynucleotide described in Section No. 0026 may be used, and furthermore, a polynucleotide acquired by the method described in Section No. 0031 can also be used. The polynucleotide may be used in its full length, or, as long as it is a polynucleotide encoding a polypeptide having the electron mediator function, it may be a polynucleotide in which the base sequence encoding the amino acid part on the C-terminal side or the N-terminal side of the polypeptide is deleted accordingly.

As a gene having a base sequence encoding glucose oxidoreductase, for example, a gene known in the art that is a base sequence encoding the glucose oxidoreductase described in Section No. 0019 may be used. For example, a sequence encoding glucose oxidase can be acquired by a common method from Aspergillus niger, a sequence encoding glucose dehydrogenase in which nicotinamide adenine dinucleotide is a coenzyme can be acquired by a common method from Bacillus megatherium, or a sequence encoding glucose dehydrogenase in which flavin adenine dinucleotide is a coenzyme can be acquired by a common method from strains of genus Aspergillus, such as Aspergillus terreus or Aspergillus oryzae (except for the RIB40 strain), genus Penicillium or genus Drosophil. As long as a polynucleotide comprising the gene is a polynucleotide encoding a polypeptide having the glucose-oxidizing function, a polynucleotide in which the base sequence encoding an amino acid sequence including part or the whole of the signal sequence part on the N-terminal side of the polypeptide is deleted may be used. The signal sequence part can be predicted using, for example, SignalP.

A gene having a base sequence encoding extracellular secretion type cytochrome and a gene having a base sequence encoding glucose oxidoreductase can be linked by a common gene manipulation method, and although there is no particular limitation regarding linking of the genes, for example, they may be linked by inserting a base sequence encoding a linker sequence between the two genes. A linker sequence may be a common linker sequence and includes, for example, a linker sequence of cellobiose dehydrogenase derived from Aspergillus terreus. Specific examples include a linker sequence in which the amino acid sequence comprises GDCSGDGGGGSGPEPVPVPDG, and two genes can be linked via a base sequence encoding the amino acid sequence.

A fusion body of the present invention can be produced using the above fusion body gene or by a peptide synthesis method such as, for example, a method similar to the method for producing extracellular secretion type cytochrome as described in Section No. 0033 to 0039.

A fusion body of the present invention produced by the production method has an absorption spectrum characteristic of extracellular secretion type cytochrome and has a function to receive an electron from glucose oxidoreductase and/or a function to give an electron to an electron acceptor as well as glucose oxidoreductase activity. Furthermore, a fusion body of the present invention has excellent electron transfer ability, and therefore, it has a feature of allowing for significant measurements of a subject to be measured with a low amount of use, characterized by a glucose oxidoreductase activity value of preferably 0.05 to 400 units, more preferably 0.1 to 200 units, or still preferably 0.2 to 100 units, in a single measurement system.

The present invention further relates to a composition for glucose measurement including an electron mediator comprising the extracellular secretion type cytochrome and glucose oxidoreductase or a fusion body. The composition may take any form such as liquid, frozen or solid by freeze drying, etc. The contents of an electron mediator comprising the extracellular secretion type cytochrome and of glucose oxidoreductase in the composition can be selected accordingly by one skilled in the art depending on the objective and the form, etc., but they are usually between approximately 0.01 and 1,000 μg/mL and 0.01 and 1,000 μg/mL, respectively. As for the ratio between an electron mediator comprising the extracellular secretion type cytochrome and glucose oxidoreductase in the composition, the molar number of the electron mediator relative to the molar number of glucose oxidoreductase is preferably less than 100 times, more preferably less than 50 times, still preferably less than 20 times, or particularly preferably less than 10 times as large. At the same time, the content of a fusion body in the composition can be selected accordingly by one skilled in the art depending on the objective and the form, etc., but it is usually between 0.01 and 1,000 μg/mL. There are no particular limitations regarding the content of the fusion body, but the amount used for a single measurement system preferably involves a glucose oxidoreductase activity value of 0.05 to 400 units, more preferably 0.1 to 200 units, or still preferably 0.2 to 100 units. Furthermore, a fusion body of the present invention is a fusion body measurable in the absence of other electron mediators such as potassium ferricyanide when the measurement range of glucose, which is a substrate, is a range larger than 5 mM or 10 mM. The composition may appropriately contain a thermostabilizing agent selected from a group comprising bovine serum albumin (BSA) or egg albumin, sugars or sugar alcohols having no active properties on glucose oxidoreductase, compounds containing a carboxyl group, alkaline-earth metal compounds, ammonium salts, sulfate salts or proteins, etc., or any other components known in the art, such as a buffering agent, in order to stabilize the electron mediator or glucose oxidoreductase or the fusion body.

The present invention relates to the use of the above electron mediator or fusion body for an enzyme electrode. An enzyme electrode can be readily created by immobilizing the above electron mediator and a glucose oxidoreductase such as glucose dehydrogenase or the above fusion body on its surface by any method known in the art.

The enzyme electrode may be used for a wide range of applications, including biosensors such as glucose sensors as well as bio-batteries.

A glucose sensor using an enzyme electrode including the electron mediator or fusion body of the present invention is a sensor for measuring the glucose concentration in a sample solution. Such a glucose sensor can be created by any method known in the art. For example, it is created by forming, on an appropriate insulating substrate, an electrode system comprising a working pole as well as a counter pole and a reference pole thereof by utilizing a method such as screen printing, and by forming an enzyme reaction layer including the above electron mediator and glucose oxidoreductase or the above fusion body on this electrode system. When a sample solution including a substrate is dropped on the enzyme reaction layer of this biosensor, the enzyme reaction layer is dissolved and the enzyme and the substrate react, which causes the electron mediator to be reduced. After the enzyme reaction is completed, the reduced electron mediator is electrochemically oxidized, and at this time, this biosensor can measure the substrate concentration in the sample solution based on the resultant oxidation current value. In addition, it is also possible to construct a biosensor that takes a system of detecting the coloring intensity or pH changes, etc.

The present invention further relates to a bio-battery including the above electron mediator and glucose oxidoreductase or the fusion body. A bio-battery of the present invention is constituted with an anode pole that performs an oxidation reaction and a cathode pole that performs a reduction reaction and is constituted by including an electrolyte separating the anode from the cathode as necessary. Using an enzyme electrode including the above electron mediator and glucose oxidoreductase or the above fusion body for the anode electrode, an electron generated by oxidizing the substrate is taken out to the electrode and a proton is simultaneously generated. At the same time, for the cathode side, an enzyme commonly used for a cathode electrode may be used, and using laccase, ascorbate oxidase or bilirubin oxidase, for example, water is generated by a reaction of the proton generated on the anode side with oxygen. For the electrode, an electrode commonly used for a bio-battery such as carbon, gold, and platinum may be used.

The present invention further relates to an activity measurement of an enzyme using the above electron mediator. In an activity measurement of an enzyme, using an enzyme, a substrate, and the electron mediator of the present invention, it is possible to find the electron-accepting state of the electron mediator by detecting spectroscopic characteristics such as absorbance or the absorbency spectrum, for example, and it is possible to perform measurements of enzyme activity. A commercially available spectroscopic measurement device may be used for this. Alternatively, in an activity measurement of an enzyme, using one or several electron mediators other than extracellular secretion type cytochrome, such as potassium ferricyanide, phenazinemethosulfate, dichlorophenolindophenol, or cytochrome C, in addition to an enzyme, a substrate, and the electron mediator of the present invention, it is possible to find the electron-accepting state of the electron mediator by detecting spectroscopic characteristics such as an absorbance or the absorbency spectrum, for example, and it is possible to perform measurements of enzyme activity.

Furthermore, various technologies used for implementing the present invention can be readily and steadily implemented by one skilled in the art based on known documents except for technologies in which their references are clearly specified. For example, genetic engineering and molecular biological technologies can be implemented based on methods described in Sambrook and Maniatis, in Molecular Cloning—A Laboratory Manual, Cold Spring Harbor Laboratory Press, New York, 1989; Ausubel, F. M. et al., Current Protocols in Molecular Biology, John Wiley & Sons, New York, N.Y., 1995, etc. or methods described in the documents referenced therein, or methods substantially similar to those or modified methods. Furthermore, the terms used in the present invention are based on the IUPAC-IUB Commission on Biochemical Nomenclature or based on the meanings of terms idiomatically used in the art. Furthermore, in the present specification, monosaccharides such as glucose refer to D-bodies unless otherwise specified, but they do not limit the present invention.

One embodiment of the present invention is a method for measuring a subject to be measured using extracellular secretion type cytochrome, an enzyme, and an electron acceptor, comprising:

-   A) a step of oxidizing the subject to be measured with the enzyme; -   B) a step of accepting an electron generated within the enzyme in     step A with the extracellular secretion type cytochrome; -   C) a step of accepting an electron in the extracellular secretion     type cytochrome generated by step B with the electron acceptor; -   D) a step of detecting changes in the electron acceptor generated by     step C; and -   E) a step of associating the quantity of changes in the electron     acceptor detected in step D with the amount or concentration of the     subject to be measured. In this manner, the affinity between     extracellular secretion type cytochrome and an enzyme is good, and     measurement of a subject to be measured can be performed with a high     sensitivity. Moreover, because extracellular secretion type     cytochrome has high water solubility, the present invention is     particularly suitable when an aqueous solution including a     water-soluble subject to be measured (e.g., a biological sample such     as blood, plasma, serum, or urine) is used in particular.     Extracellular secretion type cytochrome, an enzyme, and an electron     acceptor can be dissolved in a biological sample and used. The     present invention is a method of obtaining the amount or     concentration of a subject to be measured based on changes in an     electron acceptor after causing an electron derived from the subject     to be measured that has been transferred via an enzyme and     extracellular secretion type cytochrome to efficiently accumulate to     the electron acceptor.

For the extracellular secretion type cytochrome, the extracellular secretion type cytochrome may be used at an amount less than 100 times that of the enzyme for measurement. Moreover, extracellular secretion type cytochrome derived from any bacterium categorized as filamentous bacterium or a bacterium of genus Aspergillus, Aspergillus terreus or Aspergillus oryzae may be used. Furthermore, for the extracellular secretion type cytochrome, the electron mediator described in the Embodiment 6 may be used. A method for measuring a subject to be measured that reflects the features of extracellular secretion type cytochrome described previously can be provided.

For the enzyme, oxidoreductase or dehydrogenase may be used. Because these enzymes accept an electron from a substrate, which is a subject to be measured, they can transfer this accepted electron to the extracellular secretion type cytochrome. It is preferable to use oxidoreductase or dehydrogenase for a subject to be measured that is medically and clinically significant. Examples of the enzyme may include glucose oxidase, glucose dehydrogenase, lactate oxidase, lactate dehydrogenase, cholesterol oxidase, and cholesterol dehydrogenase. Among these examples, it is more preferable to use an enzyme that acts on glucose as the enzyme. The concentration of glucose included in a biological fluid, particularly blood and urine, becomes an important index for diagnosis, follow-up or the discovery of diabetes. In this case, glucose may be used as a subject to be measured. Moreover, as the enzyme, flavin adenine dinucleotide-dependent enzymes may be used and because these have high affinity with extracellular cytochrome and high specificity to a substrate, which is a subject to be measured, it is possible to selectively perform measurement of a subject to be measured with high sensitivity. Therefore, it is particularly suitable to use flavin adenine dinucleotide-dependent glucose dehydrogenase as the enzyme.

In the above embodiment of the present invention, an electrode may be used as the electron acceptor. In this manner, changes in the electron acceptor can be ascertained as a flow of an electron to the electrode, thereby making measurements simpler. For the flow of an electron to an electrode, the current or the amount of energization charge can be detected using a commercially available ammeter or coulomb meter, etc. As an electrode, one including gold, platinum, palladium, or carbon, etc. may be used and it is preferable to include these chemically stable materials for implementing safe measurement. Moreover, as described in the examples described later, a conductor other than an electrode or carbon powder is not necessarily required. It is believed that this is because the extracellular secretion type cytochrome of the present invention has excellent capabilities for transferring an electron to an electron acceptor including an electrode.

Moreover, for the electron acceptor, a redox compound may be used. If the spectroscopic characteristics or the amount of the compound differ between an oxidant and a reductant, it is possible to find the electron-accepting state of the compound by, for example, detecting spectroscopic characteristics such as absorbance or the absorbency spectrum, and measurements of a subject to be measured can be performed. For this, a commercially available spectroscopic measurement device may be used. Such measurements are performed as described below, for example. A liquid sample is added to a light-permeable cell including an enzyme, cellular secretion type cytochrome, and a redox compound. As a cell, for example, a commercially available cell for optical measurement made of glass or polystyrene is used. Using a commercially available spectrophotometer, light is irradiated on the cell in order to detect transmissive light. For the wavelengths of the light to be irradiated and the light to be detected, it is preferable to select wavelengths that change absorbance to a large extent due to oxidation and reduction of the redox compound. This makes it possible to use the light to detect decreases in the oxidant or increases in the reductant in the extracellular secretion type cytochrome due to oxidation of the redox compound included in the liquid sample. Furthermore, decreases in the oxidant and increases in the reductant in extracellular secretion type cytochrome can also be actualized by observing changes in the spectroscopic characteristics of the extracellular secretion type cytochrome itself, and it is also possible to measure a subject to be measured. Extracellular secretion type cytochrome in an oxidized form has an absorption peak at 419 nm, but when it is in a reduced form, it shifts to 427 nm and also presents absorption peaks at 531 and 562 nm.

Alternatively, the compound may be oxidized using a separate electrode in order to detect the flow of an electron to an electrode as a current or an amount of energization charge using a commercially available ammeter or coulomb meter, etc. As an electrode, one including gold, platinum, palladium, or carbon, etc. may be used. As an electron acceptor, a metal complex such as a ferricyanide ion and a ferrocene derivative or an organic compound such as a phenazinium derivative and a quinone derivative may be used.

It is possible to obtain changes detected in step D via a reaction of one or several redox substances. For example, changes can be seen indirectly as changes in the redox substance by further transferring an electron that has been transferred to an electron acceptor to the redox substance. Examples include a method for detecting changes in the spectroscopic characteristics of dichlorophenolindophenol using phenazinemethosulfate as the electron acceptor and dichlorophenolindophenol as the redox substance. The present embodiment is more preferable when characteristic changes in the redox substance are greater than those in the electron acceptor or when the detection resolution is high.

It is preferred that a measurable concentration of the subject to be measured is greater than 5 mM and that such an embodiment is actualized by the extracellular secretion type cytochrome of the present invention, which has high affinity between the extracellular secretion type cytochrome and an electron acceptor and a high electron transfer ability in addition to affinity between an enzyme and the extracellular secretion type cytochrome. As for other preferred concentration ranges, the concentration of the subject to be measured is measurable between 5 and 10 mM, 5 and 20 mM, 5 and 30 mM, or 5 and 40 mM. For example, if a subject to be measured is glucose in blood, it is approximately 5 mM in healthy people, but in diabetic patients, it may reach the above upper limit concentration. If only diabetic patients are targeted, a measurable concentration of the subject to be measured is greater than 10 mM. For example, it is 10 to 20 mM, 10 to 30 mM, or 10 to 40 mM. A preferred range in another embodiment is a range greater than 0.08 mM (=approximately 1.5 mg/dL). More specifically, it is 0.08 to 8 mM, 0.08 to 10 mM, 0.08 to 20 mM, 0.08 to 30 mM, or 0.08 to 40 mM. In diabetic patients, their blood glucose values can drop after the administration of insulin, but at that time, a low blood glucose value may be shown due to excessive administration. In this case, health may be damaged, and therefore, when it is necessary to measure a low concentration, the present embodiment is further preferred. The embodiment described above is actualized by the extracellular secretion type cytochrome of the present invention, which has high affinity between the extracellular secretion type cytochrome and the electron acceptor and a high electron transfer ability in addition to high affinity between an enzyme and the extracellular secretion type cytochrome.

The extracellular secretion type cytochrome and the enzyme may be fused. A method of fusion is as described in other parts of the present specification. For example, as a fusion body of the extracellular secretion type cytochrome and the enzyme, one having an amount of the extracellular secretion type cytochrome smaller than 100 times that of the enzyme may be used in a measurement. Moreover, the extracellular secretion type cytochrome derived from any bacterium categorized as a filamentous bacterium or a bacterium of genus Aspergillus, Aspergillus terreus or Aspergillus oryzae may be used. Furthermore, for the extracellular secretion type cytochrome, the electron mediator described in the Embodiment 6 may be used. In this manner, a method for measuring a subject to be measured that reflects the features of the extracellular secretion type cytochrome described previously can be provided. Because fewer materials are used for the method for measuring a subject to be measured during an operation due to the use of a fusion body, the method is more simplified. At the same time, improvements in electron transfer efficiency due to the fusion and measurements with higher sensitivity are expected.

One embodiment of the present invention is a method for measuring a subject to be measured using extracellular secretion type cytochrome, an enzyme, a first electrode, and a second electrode, comprising:

-   F) a step of oxidizing the subject to be measured with the enzyme; -   G) a step of accepting an electron generated within the enzyme in     step F with the extracellular secretion type cytochrome supported by     the first electrode; -   H) a step of accepting an electron in the extracellular secretion     type cytochrome generated by step G with the first electrode; -   I) a step of detecting a current or an energization charge amount     flowing between the first electrode and the second electrode by step     H; and -   J) a step of associating the current or the energization charge     amount detected in step I with the amount or concentration of the     subject to be measured. In this manner, the electron derived from     the subject to be measured can be detected readily and accurately by     the electrode via a catalytic reaction of the enzyme and an electron     transfer reaction by the extracellular secretion type cytochrome.     The present embodiment can be actualized by the extracellular     secretion type cytochrome of the present invention, which has high     affinity between the extracellular secretion type cytochrome and an     electron acceptor and a high electron transfer ability in addition     to the affinity between an enzyme and the extracellular secretion     type cytochrome. For the first electrode and the second electrode,     electrodes including any of carbon, gold, platinum, or palladium may     be used. For the extracellular secretion type cytochrome and an     enzyme, those described previously may be used.

The enzyme may be supported by the first electrode. In this manner, the location of a reaction is limited to the first electrode used for detection and the amount of enzyme to be used is less than when it is dissolved in a solution, and therefore, the enzyme can be utilized more efficiently. Moreover, the enzyme and the extracellular secretion type cytochrome may be fused. For the present fusion body, the one described previously may be used.

Moreover, it is preferred that the enzyme, the extracellular secretion type cytochrome, or a fusion body thereof is supported by the first electrode through a polymer molecule. In this manner, elements necessary for measurement of the subject to be measured are mostly integrated and the measurement becomes simple. Furthermore, due to the support of a polymer molecule, the output of a current or the amount of energization charge becomes large. It is believed that this is because extracellular secretion type cytochrome is densely accumulated and oriented due to the support of the polymer molecule, resulting in improvements in the efficiency of transfers of electrons from the electrode extracellular secretion type cytochrome to the electrode. Examples of polymer molecules to be used include carboxymethyl cellulose.

Step H can be induced by the application of a voltage to the first electrode. For example, the voltage of the first electrode can be applied to the second electrode by connecting a voltage control terminal of a commercially available potentiostat to the first electrode with a voltage reference terminal and an auxiliary electrode terminal to the second electrode and applying a predetermined voltage to the potentiostat. At this time, the same current as the current flowing in the first electrode is circulated in the second electrode. In such a case, it is preferred that the second electrode is oxidized and reduced. In this manner, because the current flowing in the second electrode is based on a redox reaction of the second electrode, unknown reactions in the second electrode generated in an unknown measurement sample do not occur. Because unknown reactions in the second electrode act as error factors that cannot be predicted during a measurement of a subject to be measured, measurement of a subject to be measured from which error factors have been eliminated by the present form can be implemented. Examples of the oxidizing and reducing electrode as described above include Ag/AgCl. Ag/AgCl is an oxidizing and reducing electrode in which a reaction of Ag→Ag+Cl− occurs when it is oxidized and a reaction of AgCl→Ag occurs when it is reduced. Furthermore, by using an oxidizing and reducing third electrode, application of the voltage to the first electrode may be performed to the third electrode. For example, the voltage of the first electrode can be applied to the third electrode by connecting a voltage control terminal of a commercially available potentiostat to the first electrode, an auxiliary electrode terminal to the second electrode, and a voltage reference terminal to the third electrode and by applying a predetermined voltage to the potentiostat. At this time, the third electrode is oxidized and reduced, and therefore, the oxidation and reduction reactions of the electrode maintain an equilibrium state. Thus, the voltage of the third electrode is almost constant. As a result, the voltage of the first electrode applied to the third electrode is almost constant, and therefore, measurement of a subject to be measured becomes more stable. Examples of the third electrode include Ag/AgCl.

Step F and step G may be performed concurrently. A subject to be measured, an enzyme, and extracellular secretion type cytochrome may be contacted simultaneously. In this manner, because the time required for the steps is shortened, measurement of a subject to be measured can be performed quickly. Moreover, application of a voltage to the first electrode may be performed prior to step F. This becomes feasible by applying a voltage to the first electrode using a method for applying a voltage described previously followed by bringing a subject to be measured, an enzyme, and extracellular secretion type cytochrome into contact with the first electrode. In this manner, as soon as the extracellular secretion type cytochrome accepts an electron from the enzyme, the electron is given to the first electrode, creating a form in which the extracellular secretion type cytochrome can accept an electron from the enzyme again. Therefore, the electron transfer efficiency of the extracellular secretion type cytochrome improves, allowing for measurement of a subject to be performed with high sensitivity.

Moreover, another embodiment of the present invention is a method for measuring a subject to be measured using extracellular secretion type cytochrome and an enzyme, comprising:

-   K) a step of oxidizing the subject to be measured with the enzyme; -   L) a step of accepting an electron generated within the enzyme in     step K with the extracellular secretion type cytochrome; -   M) a step of detecting changes in spectroscopic characteristics of     the extracellular secretion type cytochrome generated by step L; and -   N) a step of associating the characteristic changes detected in step     M with the amount or concentration of the subject to be measured. In     this manner, decreases in the oxidant and increases in the reductant     in the extracellular secretion type cytochrome in response to the     concentration of the subject to be measured can be detected.     Extracellular secretion type cytochrome in an oxidized form has an     absorption peak at 419 nm, but when it is in a reduced form, it     shifts to 427 nm, and additionally, it presents absorption peaks at     531 and 562 nm. Such changes can be measured using a commercially     available spectrophotometer.

Another embodiment of the present invention is an electrode for measuring the concentration or amount of a subject to be measured, wherein extracellular secretion type cytochrome and an enzyme are supported. In this manner, in addition to limiting the location of a reaction to an electrode to be used for measurement of a subject to be measured, the amount of enzyme to be used is less than when it is dissolved in a solution, and therefore, the enzyme can be utilized more efficiently. Moreover, the enzyme and the extracellular secretion type cytochrome may be fused. For the present fusion body, the one described previously may be used. In this manner, elements necessary for measurement of the subject to be measured are mostly integrated and the measurement can be easily performed by combining the electrode with another readily accessible electrode that is a second electrode, as well as a third electrode. For supporting the extracellular secretion type cytochrome and the enzyme, a polymer molecule may be used. This increases the input of a current or the amount of energization charge. It is believed that this is because extracellular secretion type cytochrome is densely accumulated and oriented due to the support of a polymer molecule, resulting in improvements in the efficiency of transfers of electrons from the electrode extracellular secretion type cytochrome to the electrode. Examples of the polymer molecule to be used include carboxymethyl cellulose.

It is preferred that the electrode includes any of carbon, gold, platinum, or palladium. Because these materials are chemically stable, measurement of a subject to be measured becomes stable. For example, an electrode can be created by dropping a polymer molecule solution including extracellular secretion type cytochrome and an enzyme on a flat-plate electrode that has any of carbon, gold, platinum, or palladium as the material and drying it.

Another embodiment of the present invention is a sensor for measuring a subject to be measured included in a sample solution, comprising at least:

-   i) an insulating first substrate; -   ii) first and second electrodes placed on the first substrate; -   iii) a reagent layer placed on the first electrode; and -   iv) a sample solution-holding part contacting the first electrode or     the reagent layer and the second electrode, wherein the reagent     layer includes extracellular secretion type cytochrome and an enzyme     is placed on either the reagent layer or the sample solution-holding     part.

The present embodiment is described in detail below with reference to the drawings.

(Skeleton Framework of the Sensor)

A sensor 1 specifically has a substrate 2, a conductive layer 3, a reagent layer 4, a spacer 5, and a cover 6. As shown in FIG. 4, the substrate 2 is a plate-like member. The substrate 2 has insulation properties. The material constituting the substrate 2 may include, for example, resins such as polyethylene terephthalate, vinyl polymer, polyimide, polyester, and styrenics, glass, and ceramics, etc.

The dimensions of the substrate 2 are not limited to any specific numbers, but, for example, the width of the substrate 2 is preferably set to 4 to 20 mm, or more preferably 5 to 10 mm. Moreover, the length of the substrate 2 is preferably set to 20 to 40 mm. Moreover, the thickness of the substrate 2 is preferably set to 0.1 to 1 mm. It is more preferable that all of the width, length, and thickness of the substrate 2 are within these ranges.

As shown in FIG. 4, the conductive layer 3 is formed with a substantially even thickness on the substrate 2. The conductive layer 3 includes three electrodes 31 to 33. The electrode 31, which is a first electrode, may be referred to as a working electrode, the electrode 32, which is a second electrode, may be referred to as a counter electrode, and the electrode 33 may be referred to as a detection electrode. Furthermore, the detection electrode 33 may be omitted.

A portion of each of the electrodes 31 to 33 is placed so that it faces a capillary 51. Other portions of the electrodes 31 to 33 are exposed without being covered with the spacer 5 or the cover 6 at the end opposite from an inlet 52 of the sensor 1. This exposed part functions as a lead and receives the application of a voltage from a measuring instrument 101 or transmits a current to the measuring instrument 101.

Each electrode may be formed by printing using a conducting material, etc. or may be formed by forming a nonconductive track by laser ablation, etc. after covering the substrate 2 with a conducting material. For example, as a non-attributive example, the conductive layer 3 may be formed by sputtering palladium on the substrate 2 and a nonconductive track may be formed by laser ablation. A nonconductive track preferably has a width of 0.01 to 0.5 mm, or more preferably 0.05 mm to 0.3 mm.

Furthermore, the constituent material of the conductive layer 3 may be a conducting material (conducting substance) and is not particularly limited. Examples of the conducting material may include an inorganic conducting substance represented by a metal, metal mixture, alloy, metal oxide, and metal compound, etc., an organic conducting substance such as a conductive hydrocarbon polymer or a heteroatom-containing conductive polymer, etc., or a combination of these substances. As the constituent material of the conductive layer 3, palladium, gold, platinum, and carbon, etc. are preferred and palladium is particularly preferred. Because these materials are chemically stable, they stably function as an electrode, resulting in stable measurement of a subject to be measured.

The thickness of the conductive layer 3 may be changed according to the method for forming the layer and the constituent material. For example, if the conductive layer 3 is formed by sputtering, the thickness of the conductive layer 3 is preferably 0.1 to 20 nm, or more preferably 1 to 10 nm. If the conductive layer 3 is formed by printing, the thickness of the conductive layer 3 is preferably 0.1 to 50 μm, or more preferably 1 to 30 μm.

An oxidizing and reducing substance may be applied to a part corresponding to the electrode 32 (second electrode) of the conductive layer 3, and this may be the electrode 32. In this manner, the current flowing in the second electrode is based on a redox reaction of the second electrode, and therefore, unknown reactions in the second electrode generated in an unknown measurement sample do not occur. Unknown reactions in the second electrode act as error factors that cannot be predicted during measurement of a subject to be measured, and therefore, measurement of a subject to be measured from which such error factors are eliminated by the present form can be implemented. Examples of the oxidizing and reducing electrode as described above include Ag/AgCl. Ag/AgCl is an oxidizing and reducing electrode in which a reaction of Ag→Ag⁺Cl⁻ occurs when it is oxidized and a reaction of AgCl→Ag occurs when it is reduced.

As shown in FIG. 4, the reagent layer 4 is placed so as to contact at least the electrode 31 (first electrode). Moreover, a form in which the reagent layer 4 is placed only on the electrode 31 is preferred. In this manner, the location of a reaction of a subject to be measured is limited to the first electrode used for detection and the enzyme can be utilized more efficiently. Further, a reduction reaction in the electrode 32, which is the second electrode, can be performed independently from the first electrode. Thus, it becomes easier to use the oxidizing and reducing electrode described above as the second electrode.

The reagent layer 4 functions as an active part of the sensor 1 along with the electrodes 31 and 32. The active part is a region that is electrochemically active and is a part that generates an electrical signal in response to a particular substance in a liquid sample. Specifically, the reagent layer 4 includes an enzyme and/or extracellular secretion type cytochrome.

In the reagent layer 4, one type or more types of enzymes are included. An enzyme included in the reagent layer 4 is specifically an enzyme in which a subject to be measured is a substrate, and in particular, it is preferred that it is an enzyme that reacts specifically to the subject to be measured. The enzyme gives an electron to the extracellular secretion type cytochrome in response to the concentration of the subject to be measured (i.e., the amount of reaction with the subject to be measured).

As an enzyme included in the reagent layer 4, oxidoreductase is particularly preferred. Oxidoreductase specifically includes oxidase and dehydrogenase in which the subject to be measured is a substrate. Specific examples of these oxidoreductases may include glucose oxidase and glucose dehydrogenase if the subject to be measured is glucose sugar, lactate oxidase or lactate dehydrogenase if the subject to be measured is lactic acid, cholesterol esterase or cholesterol oxidase if the subject to be measured is cholesterol, alcohol oxidase if the subject to be measured is alcohol, and bilirubin oxidase if the subject to be measured is bilirubin.

Regarding the enzymes, there are no particular limitations regarding the coenzyme dependency thereof. For example, an enzyme included in the reagent layer 4 may be an enzyme having dependency on a coenzyme such as NAD (nicotinamide adenine dinucleotide), NADP (nicotinamide adenine dinucleotide phosphate), PQQ (Pyrroloquinoline quinone) or FAD (flavin adenine dinucleotide), etc.

It is preferred that a coenzyme of an enzyme is FAD or PQQ. In an enzyme corresponding to these coenzymes, the coenzyme binds to its enzyme protein or is included therein. Therefore, in implementing manufacturing and measurement methods for the sensor, it is not necessary to add a coenzyme separately from the enzyme. Consequently, the constitution, production process, and measurement process of the sensor can be simplified.

In the case of NAD and NADP-dependent enzymes, it is necessary to separately add coenzymes NAD and NADP, which function in a state where they do not bind to enzyme proteins, along with an enzyme. The constitution and process are more complex compared to enzymes in which FAD and PQQ are coenzymes, but they are feasible in the invention of the present application.

For example, the enzymes may be FAD-dependent oxidase and NAD-dependent, PQQ-dependent or FAD-dependent dehydrogenase, etc. Specific examples of oxidase and dehydrogenase are described above.

An enzyme in the reagent layer 4 may be fused to the extracellular secretion type cytochrome, and for the present fusion body, the one described previously may be used. Moreover, for the extracellular secretion type cytochrome, the one described previously may be used.

The enzyme content of the reagent layer 4 is set to a degree at which detection of a target substance is possible and is preferably set to approximately 0.2 to 20 U (unit), or more preferably 0.5 to 10 U, per single measurement or per sensor 1.

Moreover, in the reagent layer 4, a coenzyme that matches the enzyme may be included.

The reagent layer 4 includes extracellular secretion type cytochrome. Extracellular secretion type cytochrome can reversibly become an oxidant and a reductant and mediates electron transfers between substances directly or in cooperation with another substance. For example, if an enzyme that oxidizes a substrate is included in the reagent layer 4, the enzyme receives an electron from the substrate by oxidizing the substrate and gives the electron to the coenzyme. Consequently, the coenzyme becomes a reductant from an oxidant. The extracellular secretion type cytochrome, which is an oxidant, receives the electron from the coenzyme that has become a reductant and restores the coenzyme as an oxidant. Consequently, the extracellular secretion type cytochrome becomes a reductant. The extracellular secretion type cytochrome that has become a reductant gives the electron to the electrode 31 or 32 and becomes an oxidant. In this manner, the extracellular secretion type cytochrome mediates electron migration between the enzyme and the electrode.

The above coenzyme may be retained by an enzyme protein by binding to the enzyme protein (enzyme molecule). Moreover, the coenzyme may exist in a solution separately from the enzyme protein.

The reagent layer 4 may include components other than an enzyme and extracellular secretion type cytochrome. As such components, various substances capable of enhancing the storage stability of an enzyme or extracellular secretion type cytochrome or enhancing the responsiveness between an enzyme and a target substance are used. Such components may include, for example, a buffering agent.

The reagent layer 4 may be formed by various methods. The forming method may include, for example, a printing method and a coating method, etc.

An example of a forming method is described below. The reagent layer 4 may be formed by dropping a constant amount of an aqueous solution including an enzyme, extracellular secretion type cytochrome, and other components as necessary on the electrode 31 using a microsyringe, etc. before leaving it to stand and performing desiccation in an appropriate environment. Furthermore, if a wider surface of the electrode 31 is covered with the reagent layer 4, the dropped aqueous solution may be spread with the tip of a syringe, etc.

The amount of aqueous solution to be dropped is not limited to a specific number, but it is preferably 0.5 to 5 μL, or more preferably 1 to 2 μL.

The shape of the reagent layer 4 is not limited to any specific shape. This shape may be rectangular or circular, etc. The area of the reagent layer 4 (area in the planar direction of the substrate 2) is determined according to the properties and size of the device. This area may be preferably 1 to 25 mm², or more preferably 2 to 10 mm².

The respective content amounts of the enzyme, the extracellular secretion type cytochrome, and the other components to be applied are selected according to the properties and size of the device required.

As shown in FIG. 4, the spacer 5, which is a second substrate, is a member for forming a sample solution-holding part including a subject to be measured.

Specifically, the spacer 5 is a plate-like member and covers the entire conductive layer 3 except for the lead parts of the electrodes 31 to 33 and a capillary 51 part described later. The spacer comprises a rectangular notch to cause an end opposite from the lead parts of the electrodes 31 to 33 to be exposed. The notch part may be U-shaped. Because the spacer 5 comprises this notch, a sample solution-holding part is formed. Furthermore, part of the sample solution-holding part is formed with the cover 6 by sticking the cover 6, which is a third substrate, to the spacer 5. As described above, the capillary 51 that functions as the sample solution-holding part surrounded by the spacer 5, the conductive layer 3, and the cover 6 is formed. As described above, the spacer 5 can provide side walls of the capillary 51 and further define the length, width and height of the capillary 51.

The capacity of the capillary 51 is preferably set to approximately 0.1 to 1.0 μL (microliter). The thickness of the spacer 5 is preferably 0.1 to 0.2 mm, the length of the notch composed by the spacer is preferably 1 to 5 mm, and the width composed by the spacer is preferably 0.5 to 2 mm. Furthermore, these dimensions may be selected accordingly so that the capillary 51 has a desirable capacity. For example, if the spacer 5 with a thickness of 0.145 mm comprising a notch with a length of 3.4 mm and a width of 1.2 mm, the capillary 51 is provided with a length of 3.4 mm, a width of 1.2 mm, a height of 0.145 mm, and a capacity of 0.6 μL.

The capillary 51 sucks in a liquid sample from an inlet 52, which is its opening, with capillary action and retains the sample on the electrodes 31 to 33.

As shown in FIG. 4, the cover 6 is a plate-like member covering the entire spacer 5. The cover 6 comprises a hole penetrating from the surface to the back side. This hole functions as a ventilation hole 61 leading to the outside from the capillary 51. The ventilation hole 61 is an exhaust hole for discharging air within the capillary 51 to the outside when a liquid sample is sucked in the capillary 51. Discharging air in this manner facilitates the sucking in of the liquid sample into the capillary 51. It is preferred that the ventilation hole 61 is provided at a position away from the inlet 52 (i.e., the rear of the capillary 51 when viewed from the inlet 52). By placing the inlet 52 in this manner, a liquid sample can be quickly transferred to the rear of the capillary 51 from the inlet 52.

Furthermore, in the above embodiment, an example in which the first and second electrodes are placed on the same substrate has been described, but the present invention is not limited to this. For example, one electrode may be placed on the substrate and the other one may be placed on the cover substrate.

The abovementioned sensor 1 is used in a measurement system 100 as shown in FIG. 5. The measurement system 100 has the sensor 1 and a measuring instrument 101.

As shown in FIG. 5 and FIG. 6, the measuring instrument 101 comprises a display part 102, an applied part 103, a switching circuit 107, a reference voltage source 108, a current/voltage converting circuit 109, an A/D converting circuit 110, and a computing part 111. The measuring instrument 101 further has a connector corresponding to each electrode of the sensor 1. In FIG. 6, connectors 104 to 106 are depicted.

The display part 102 displays the state of the measuring instrument 101, measurement results, and operational details, etc. The display part 102 is specifically actualized by a liquid crystal display panel.

As shown in FIG. 5, the sensor 1 is attachably inserted into the applied part 103.

As shown in FIG. 6, the connectors 104 to 106 are respectively connected to the electrodes 31 to 33 of the sensor 1 by attaching the sensor 1 to the applied part 103.

The switching circuit 107 connects the connectors 104 to 106 to the reference voltage source 108 or to the current/voltage converting circuit 109.

The reference voltage source 108 applies a voltage to the electrodes 31 to 33 via the connectors 104 to 106.

The current/voltage converting circuit 109 receives a current from the sensor 1 via the connectors 104 to 106, converts the current into a voltage, and outputs to the A/D converting circuit 110.

The A/D converting circuit 110 converts the output value (analog value) from the current/voltage converting circuit 109 to a pulse (digital value).

The computing part 111 has a CPU (Central Processing Unit) and recording media such as a ROM (Read Only Memory) and a RAM (Random Access Memory). The computing part 111 performs calculations of the concentration of a target substance or controls the movement of each part within the measuring instrument 101.

The concentration calculation function of the computing part 111 will be described. In a storage medium of the computing part 111, a conversion table used for determining the concentration of a target substance in a sample and a correction-amount table used for determining the correction amount of this concentration, etc. are stored. The computing part 111 calculates a tentative concentration of the target substance with reference to the conversion table based on a pulse from the A/D converting circuit 110. The computing part 111 further determines the final concentration of the target substance using a correction amount in the correction amount table. The concentration calculated in this manner is displayed on the display part 102.

Moreover, in addition to the concentration calculation function, the computing part 111 controls switching of the switching circuit 107, controls the voltage of the reference voltage source 108, measures the concentration and measures the time when selecting a correction amount (timer function), outputs display data to the display part 102, and has a communication function with external devices, etc.

Various functions of the computing part 111 are actualized by the CPU reading out and executing a program stored in the ROM, etc., which is not shown.

Concentration measurement by the measurement system 100 will be described below.

Once the sensor 1 is inserted into the applied part 103, the connectors 104 to 106 are connected to the electrodes 31 to 33, respectively. Moreover, a switch (not shown) within the applied part 103 is pressed down by the sensor 1. Due to the switch being pressed down, the computing part 111 judges that the sensor 1 has been attached and causes the measuring instrument 101 to enter a sample standby state. The sample standby state is a state in which, under the control of the computing part 111, the reference voltage source 108 has started the application of a voltage between the working electrode 31 and the detection electrode 33 via the connectors 104 and 106 while the current/voltage converting circuit 109 has started current measurement, wherein the liquid sample has not been subjected to a measurement.

Once the user causes a liquid sample to be attached to the inlet 51 of the sensor 1, the liquid sample is drawn into the capillary 52 from the inlet 51 with capillary action.

Examples of the liquid sample may include liquid samples derived from a living organism, such as blood, sweat, or urine, liquid samples derived from the environment, and liquid samples derived from food. For example, if the sensor 1 is used as a blood glucose value sensor, the user punctures his/her own finger, palm or arm to squeeze out a small amount of blood and uses this blood as a liquid sample for measurement in the sensor 1.

When the liquid sample reaches the working electrode 31 and the detection electrode 33, the current value that the computing part 111 receives via the current/voltage converting circuit 109 changes. Based on this change, the computing part 111 judges that the liquid sample has been sucked in by the sensor 1. Once the suction of the liquid sample is detected in this manner, measurement is started.

Within the sensor 1, the liquid sample, the enzyme, and the mediator contact each other on the electrodes 31 and 32.

Based on the control of the computing part 111, the switching circuit 107 connects the connector 104 and the connector 105 to the reference voltage source 108 and the current/voltage converting circuit 109. In this manner, a voltage is applied between the working electrode 31 and the counter electrode 32 and the current generated between the working electrode 31 and the counter electrode 32 is transferred to the current/voltage converting circuit 109.

The current that has flowed to the current/voltage converting circuit 109 is converted into a voltage. Subsequently, this voltage is further converted into a pulse by the A/D converting circuit 110. The computing part 111 calculates the concentration of a specific component based on this pulse. The value calculated by the computing part 111 is displayed on the display part 202. At that time, other information may be displayed for the user as well.

After the measurement is completed, the user can remove the sensor 1 from the applied part 103.

Furthermore, the reference voltage source 108 is configured to provide a voltage sufficient to cause an intended electrochemical reaction between the two electrodes 31 and 32. This voltage is principally set according to the chemical reaction and the electrode that are used.

Moreover, the above (f) is executed by, for example, a computing device calculating the concentration of a target substance using a calibration curve obtained using a standard solution in which the concentration of the target substance is given.

Another embodiment of the present invention is an electrode for measuring the concentration or amount of a subject to be measured, wherein extracellular secretion type cytochrome and an enzyme are supported. In this manner, in addition to limiting the location of a reaction to an electrode to be used for a measurement of a subject to be measured, the amount of enzyme to be used is less than when it is dissolved in a solution, and therefore, the enzyme can be utilized more efficiently. Moreover, the enzyme and the extracellular secretion type cytochrome may be fused. For the present fusion body, the one described previously may be used. In this manner, elements necessary for measurement of the subject to be measured are mostly integrated and the measurement can be easily performed by combining the electrode with another readily accessible electrode that is a second electrode, as well as a third electrode. For supporting the extracellular secretion type cytochrome and the enzyme, a polymer molecule may be used. This increases the input of current or the amount of energization charge. It is believed that this is because extracellular secretion type cytochrome is densely accumulated and oriented due to the support of a polymer molecule, resulting in improvements in the efficiency of electron transfers from the electrode extracellular secretion type cytochrome to the electrode. Examples of the polymer molecule to be used include carboxymethyl cellulose.

The present invention is described below in more detail in accordance with the examples. Furthermore, the technical scope of the present invention is not limited by these descriptions. The contents described in the documents referenced in the present specification constitute part of the present specification as the disclosed content of the present specification.

EXAMPLE 1 Acquisition of an Extracellular Secretion Type Cytochrome Gene

1) Culture

First, 150 mL of a liquid medium consisting of Pinedex 2% (manufactured by Matsutani Chemical Industry co., Ltd.) (W/V), triptone 1% (manufactured by BD) (W/V), potassium dihydrogen phosphate 0.5% (manufactured by Nacalai Tesque) (W/V), magnesium sulfate heptahydrate 0.05% (W/V) (manufactured by Nacalai Tesque) and water was poured into a 500-mL Sakaguchi flask, which was stopped up with a silicosen, and subsequently autoclaved at 121° C. for 20 minutes. This liquid medium that had been cooled was inoculated with an Aspergillus terreus NIH2624 strain or an Aspergillus oryzae RIB40 strain and was cultured while being shaken at 30° C. for 62 hours.

2) Total RNA Extraction

After 2 g each of a wet fungus body of the Aspergillus terreus NIH2624 strain or the Aspergillus oryzae RIB40 strain cultured by the method described in Example 1-1) was frozen with liquid nitrogen and crushed, 0.1 mg each of Total RNA was re-extracted using illustra RNAspin Mini Kit (manufactured by GE Healthcare Japan).

3) Preparation of a cDNA Library

From the respective Total RNA of the Aspergillus terreus NIH2624 strain and the Aspergillus oryzae RIB40 strain, the respective cDNA libraries were prepared by a reverse transcription reaction using a reverse transcriptase enzyme and an oligo dT primer with an adapter. For the reaction, Prime Script RT-PCR Kit (Manufactured by Takara Bio Inc.) was used, and for the reaction conditions, the protocol described in the operating manual was followed.

4) Subcloning of an Extracellular Secretion Type Cytochrome Gene to Escherichia coli

Two pairs of primers shown in Table 1 below were synthesized, and with the cDNA library of the Aspergillus terreus NIH2624 strain or the Aspergillus oryzae RIB40 strain as a template, the respective extracellular secretion type cytochrome genes were subjected to PCR amplification from the primers.

Furthermore, the primers in Table 1 were designed based on the sequences of XM_(—)001216771 (Aspergillus terreus: believed to be encoding a protein at a part of mRNA) and XM_(—)001820457 (Aspergillus oryzae: believed to be encoding a protein at a part of mRNA) from the gene database published in NCBI (National Center for Biotechnology Information) (website http://www.ncbi.nlm.nih.gov/). This is because the results of performing a domain structure prediction of the above XM_(—)001216771 using SMART (website http://smart.embl-heidelberg.de/) led to the conclusion that it was a protein having electron transfer ability (cytochrome). Furthermore, a sequence recognized by a restriction enzyme BglII (within the rectangular frame) was added to the forward side (AT Cytb Bgl2_F, AO Cytb Bgl2_F) and a sequence recognized by XhoI or NcoI (within the rectangular frame) was added to the reverse side (AT Cytb Xho1_R, AO Cytb Nco1_R).

TABLE 1

PCR was performed with a combination of AT Cytb Bgl2_F and AT CytbXho1_R for the Aspergillus terreus NIH2624 strain and with a combination of AO Cytb Bgl2_F and AO Cytb Nco1_R for the Aspergillus oryzae RIB40 strain, and the respective intended gene regions were amplified. Furthermore, for the PCR, commercially available polymerase pfu ultra (manufactured by STRATAGENE) was used and the reaction conditions were [94° C./2 minutes→(94° C./30 seconds→55° C./30 seconds→72° C./1 minute)×30 cycles].

Next, an amplified gene fragment derived from the Aspergillus terreus NIH2624 strain was broken with the restriction enzymes BglII and XhoI and an amplified gene fragment derived from the Aspergillus oryzae RIB40 strain was broken with the restriction enzymes BglII and NcoI, and they were ligated to vectors for expression with fungi similarly subjected to restriction enzyme treatment with BglII and XhoI or BglII and NcoI, respectively, in order to construct vectors for the expression of extracellular secretion type cytochrome derived from the Aspergillus terreus NIH2624 strain or the Aspergillus oryzae RIB40 strain, respectively. Furthermore, for the present vectors, using an improved promoter of the glucoamylase system derived from Aspergillus oryzae that is described in the known Document 1 (Heterologous gene expression system of genus Aspergillus, MINETROKI Toshitaka, Chemistry & Biology, 38, 12, P831-838, 2000), vectors capable of expressing the intended gene were prepared. The above vectors for the expression of extracellular secretion type cytochrome were transformed by each being introduced into Escherichia coli JM109 strains. Using an illustra plasmidPrep Midi Flow Kit (manufactured by GE Healthcare Japan), plasmid was extracted from three clones of each transformant obtained and a sequence analysis of the insert was performed, and consequently, the intended genes could be confirmed in all of the plasmids. 5) Acquisition of an Extracellular Secretion Type Cytochrome Gene Derived from an Aspergillus terreus NIH2624 Strain

However, the acquired gene derived from the Aspergillus terreus NIH2624 strain had 10 bases deleted compared to a publicized sequence (SEQ ID NO: 1), and therefore, 275 surrounding bases including the deleted bases were artificially synthesized in order to substitute the bases of the extracellular secretion type cytochrome gene derived from the Aspergillus terreus NIH2624 strain. The created gene fragment was cloned into Escherichia coli similarly with the method described in Example 1-4) and was then subjected to gene analysis, and consequently, a gene with a sequence identical to that of the publicized sequence XM_(—)001216771 could be acquired.

Furthermore, the gene sequence and amino acid sequence of the extracellular secretion type cytochrome derived from the Aspergillus terreus NIH2624 strain and the Aspergillus oryzae RIB40 strain acquired by the present invention are shown in SEQ ID NO: 1 and 2 and SEQ ID NO: 3 and 4, respectively.

EXAMPLE 2 Expression and Purification of Extracellular Secretion Type Cytochrome

1) Transformation of a Fungus and Confirmation of Expression of an Intended Protein

Using the vector for expression of extracellular secretion type cytochrome derived from the Aspergillus terreus NIH2624 strain or the vector for expression of extracellular secretion type cytochrome derived from the Aspergillus oryzae RIB40 strain that were prepared in Examples 1-4) or 1-5), recombinant fungi (Aspergillus oryzae) producing each extracellular secretion type cytochrome were created in accordance with the methods described in the known Documents 1 and 3 (Gene manipulation technology for rice molt fungi for refilled sake, GOMI Katsuya, Journal of the Brewing Society of Japan, P494-502, 2000). Furthermore, for the host fungi to be used, as described in the known Document 2 (BioSci. Biotech. Biochem., 61(8), 1367-1369, 1997), those that were bred at a brewing experiment station in 1997 (Heisei 9), utilized for analyses of transcription factors and for breeding of high-producing strains of various enzymes, etc., and subdivided are available. After each transformant was selected in a Czapek-Dox solid medium, each transformant was inoculated in a thick test tube (22 mm×200 mm) in which 10 mL each of the liquid medium described in Example 1-1) had been poured, and it was cultured while being shaken at 30° C. for 62 hours. After the culturing was completed, each culture solution was centrifuged (3,000×g, 20 minutes) and those from which deposition was removed were determined to be crude protein samples. Each crude protein sample was subjected to SDS-polyacrylamide gel electrophoresis (SDS-PAGE) using 15.0% polyacrylamide gel in accordance to the method of LaemmLi, et al. After migration, it was stained with Coomassie brilliant blue (CBB) and the expression of each intended protein was confirmed by comparing its mobility with that of a molecular weight marker (LMW Marker manufactured by GE Healthcare Japan), and consequently, the expression of an intended protein with a molecular weight of approximately 30 kDa was confirmed in all of the samples.

2) Seed Culturing of a Recombinant Fungus

Each recombinant fungus created in Example 2-1) was inoculated in a Sakaguchi flask described in Example 1-1) and cultured while being shaken at 30° C. for 62 hours in order to obtain various culture solutions.

3) Actual Culture

First, 3.5 L of each liquid medium consisting of Pinedex 2% (manufactured by Matsutani Chemical Industry co., Ltd.) (W/V), triptone 1% (manufactured by BD) (W/V), potassium dihydrogen phosphate 0.5% (manufactured by Nacalai Tesque) (W/V), magnesium sulfate heptahydrate 0.05% (W/V) (manufactured by Nacalai Tesque), an antifoam agent, and water was prepared to pH 6.0, poured into a 5-L jar fermentor, and autoclaved at 121° C. for 20 minutes. These liquid media that had been cooled were each inoculated with 45 mL of each culture solution prepared in Example 2-2), and they were cultured at 30° C. for 62 hours under conditions of aeration and agitation. The culture supernatant obtained by filtering each culture solution was used as the crude protein solution. From the crude protein solution, each extracellular secretion type cytochrome was further isolated and purified by the following steps 4)-6).

4) Concentration/Desalination

Each crude protein solution was concentrated with an ultrafiltration membrane “Pellicon 2 module” (manufactured by Millipore K.K.) with a molecular weight cut-off of 10,000 and substituted into a 20-mM potassium phosphate buffer solution (pH7.0) in order to prepare a concentrated solution of each crude protein.

5) Purification with Butyl-TOYOPEAL650M (Manufactured by Tosoh Corporation)

Each supernatant obtained by preparing each of the above concentrated solutions of the crude protein to a 60% saturated ammonium sulfate solution (pH7.5) and centrifuging each solution was passed through a Butyl-TOYOPEAL650M column equilibrated with a 20-mM potassium phosphate buffer solution (pH7.0) including 60% ammonium sulfate in order to cause each intended protein to be absorbed, and after it was washed with the same buffer solution, each protein was eluted by a gradient elution method at an ammonium sulfate concentration of 60% to 30%. Regarding each intended fraction, a spectral analysis of 350 nm to 600 nm was performed and those in which a reduction spectrum of 562 nm, which is characteristic of cytochrome b562, was observed were collected.

6) Purification with DEAE-Cellulofine A-500 (Manufactured by Seikagaku Corporation)

Each of the above collected fractions was concentrated with an ultrafiltration membrane “Pellicon 2 module,” and after desalination, they were each equilibrated with a 5-mM Tris-HCL buffer solution (pH 8.0), caused to be absorbed by DEAE-Cellulofine A-500 equilibrated with the above buffer solution, and washed with the same buffer solution, and subsequently, each protein was respectively eluted by a gradient elution method with the same buffer solution and the same buffer solution including 0.2 M NaCl in order to collect an intended fraction. Each of the obtained purified proteins was subjected to SDS-PAGE using 15.0% polyacrylamide gel in accordance with the method of LaemmLi, et al. After migration, they were stained with CBB, and the results of comparing the mobility with that of a molecular weight marker (LMW Marker manufactured by GE Healthcare) confirmed that they were single (molecular weight of approximately 30 kDa). As described above, the extracellular secretion type cytochrome derived from the Aspergillus terreus NIH2624 strain or the extracellular secretion type cytochrome derived from the Aspergillus oryzae RIB40 strain could be readily collected from the culture supernatant of each transformant, stably purified, and isolated. Furthermore, by establishing a production method using Aspergillus oryzae, a bacterium belonging to the genus Aspergillus, which is a filamentous bacterium, as a host made it possible to obtain a recombinant extracellular secretion type cytochrome with no excessive glycosylation compared to a production method using yeast as a host.

Furthermore, the extracellular secretion type cytochrome derived from the Aspergillus terreus NIH2624 strain (AtCytb) or the extracellular secretion type cytochrome derived from the Aspergillus oryzae RIB40 strain (AoCytb) that was obtained in Example 2-6) had the following physicochemical properties.

-   (1) Having a function to receive an electron from glucose     oxidoreductase and/or a function to give an electron to an electron     acceptor. -   (2) Molecular weight: Approximately 30 kDa (subunit molecular weight     when extracellular secretion type cytochrome recombined with a     filamentous bacterium into which the polynucleotide described in SEQ     ID NO: 1 or SEQ ID NO: 3 has been introduced is subjected to     polyacrylamide gel electrophoresis (SDS-PAGE).)     Furthermore, regarding the above molecular weight, because a sugar     chain is originally added to the present enzyme, when the manner in     which the sugar chain is attached changes according to the culture     conditions or purification conditions, the molecular weight differs,     and the sugar chain or amino acid to be added also changes according     to the type, etc. of transformed cell or vector system and the     molecular weight differs. Furthermore, if the amino acid sequence     length or the manner in which the sugar chain is attached changes     according to the type of polynucleotide to be introduced, the     molecular weight differs. -   (3) Presenting a red color. -   (4) Having absorption spectra characteristic of cytochrome b562 in a     reduced form (427 nm, 531 nm, and 562 nm). -   (5) Being a soluble protein.

EXAMPLE 3 Measurement of a Response Current Value for Glucose

The configuration of an enzyme electrode was that an enzyme and an electron mediator were immobilized on the surface of glassy carbon (diameter of 6 mm, diameter of the electrode surface). The enzyme electrode was equilibrated by soaking in a 1 M potassium phosphate buffer solution (pH 7.0) before use. For the enzyme, glucose dehydrogenase derived from Aspergillus terreus (AtGLD, 2,000 U/mg) as described in International Publication No. 2006/101239 and glucose dehydrogenase derived from Aspergillus oryzae (AoGLD, 2,000 U/mg) as described in International Publication No. 2008/001903 were used, and for the electron mediator, AtCytb obtained in Example 2, AoCytb, cytochrome C derived from hoarse myocardium (HCytC), cytochrome b derived from Escherichia coli (EcCytb), and potassium ferricyanide were used in order to create an enzyme electrode. In the enzyme electrode, the content amount of the enzyme was equivalent to 5 U and the content amounts of the electron mediators were equivalent to 0.28×10⁻¹⁰ mol ((molar ratio) enzyme:electron mediator=1:1).

The response properties were studied based on the results of measuring response currents of a plurality of glucose solutions with different concentrations. The response current value was measured by soaking the enzyme electrode, the reference electrode, and the counter pole in a reaction tank retaining a glucose solution adjusted to the intended concentration, simultaneously applying a voltage between the enzyme electrode and the counter pole, and defining the reference electrode as a referential electrode. The glucose solution was created by dissolving glucose in a 1 M potassium phosphate buffer solution (pH7.0). The concentration of the glucose solution was set to 0.1 mM to 40 mM. for the reference electrode, an Ag/AgCl electrode was used, and for the counter pole, a Pt electrode was used. The applied voltage value was +500 mV and measurement of the response current value was performed while maintaining the temperature of the reaction tank at 30° C. The results of the measurement of the response current values are shown in Table 2 and Table 3 as well as FIG. 1 and FIG. 2.

TABLE 2 {circle around (1)} AoCytb + AtGLD {circle around (2)} AtCytb + AtGLD {circle around (3)} HCyt + AtGLD (5U) (5U) (5U) glc conc. (mM) nA glc conc. (mM) nA glc conc. (mM) nA 10 74.8 10 28.5 10 14.8 20 86.9 20 35.1 20 17.8 30 93.9 30 40.8 30 18.9 40 110.2 40 46.9 40 19 {circle around (5)} Potassium {circle around (4)} AtGLD (5U) Ferricyanide + {circle around (6)} EcCytb + AtGLD only AtGLD (5U) (5U) glc conc. (mM) nA glc conc. (mM) nA glc conc. (mM) nA 10 14.5 10 12.9 10 15.7 20 14.5 20 13.2 20 16.6 30 14.1 30 12.8 30 16.1 40 13.8 40 12.2 40 16.4

TABLE 3 {circle around (1)} AoCytb + AoGLD {circle around (2)} AtCytb + AoGLD {circle around (3)} HCyt + AoGLD (5U) (5U) (5U) glc conc. (mM) nA glc conc. (mM) nA glc conc. (mM) nA 0 13.0 0 5.9 0 7.3 0.1 21.8 0.1 19.6 0.1 12.2 0.2 28.4 0.2 24.4 0.2 15.9 0.5 39.3 0.5 35.0 0.5 18.9 1 48.1 1 37.9 1 21.1 5 51.1 5 40.4 5 20.9 10 51.3 10 46.5 10 22.3 20 55.1 20 53.6 20 23.6 30 59.1 30 55.1 30 22.7 40 62.7 40 55.6 40 23.7 {circle around (5)} Potassium {circle around (4)} AoGLD (5U) ferricyanide + {circle around (6)} EcCytb + AtGLD only AoGLD (5U) (5U) glc conc. (mM) nA glc conc. (mM) nA glc conc. (mM) nA 0 6.5 0 6.5 10 19.2 0.1 10.1 0.1 10.1 20 20.3 0.2 10.5 0.2 10.6 30 22.7 0.5 11.1 0.5 11 40 23.7 1 11.5 1 11.5 5 11.5 5 11.5 10 11.8 10 10.6 20 12.3 20 10.3 30 12.5 30 10.9 40 12.7 40 11.2

As will be appreciated from these results, it was shown that in the enzyme electrodes involving the use of no electron mediators or the enzyme electrodes involving the use of HCytC, EcCytb or potassium ferricyanide, the increased amounts of the response current values due to the increase of the glucose concentration were very small and appropriate electron transfer was not performed between the enzymes and the electrodes. In contrast, in the enzyme electrodes involving the use of AoCytb or AtCytb, the increased amounts of the response current values due to the increase of the glucose concentration were large and good response properties were obtained. In particular, significant response properties were obtained in the high-glucose-concentration region when AtGLD was used and in the low-glucose-concentration region when AoGLD was used. As described above, in enzyme electrodes in which the acquired extracellular secretion type cytochrome was used as an electron mediator, measurement with a glucose concentration of up to 40 mM could be performed by an electrode method using extracellular secretion type cytochrome that was equimolar to the molar number of glucose oxidoreductase and without using an electron mediator such as a metal complex, and an electron mediator comprising extracellular secretion type cytochrome with high affinity with glucose oxidoreductase was obtained.

Measurement method: Amperometry

Conditions: Init E=0.5 V, Smpl Intvl=1, Run Time=1400 sec.

Buffer and electrolyte concentration: 1 M KPB+0.1 M KCl

EXAMPLE 4 Acquisition of a Fusion Body Gene

1) Acquisition of an Extracellular Secretion Type Cytochrome Gene

With the amplified gene fragment derived from the Aspergillus terreus NIH2624 strain acquired in Example 1-4) as Template A or the amplified gene fragment derived from the Aspergillus oryzae RIB40 strain as Template B, PCR was performed with the following reaction conditions. As a result of performing PCR using primers AtC-Kpn-F and AtC-R for Template A, an extracellular secretion type cytochrome (AtCytb) gene fragment derived from an Aspergillus terreus NIH2624 strain was acquired. At the same time, as a result of performing PCR using primers AoC-Kpn-F and AoC-R for Template B, an extracellular secretion type cytochrome (AoCytb) gene fragment derived from an Aspergillus oryzae RIB40 strain was acquired. Furthermore, the primer AtC-Kpn-F of the forward side is a primer designed to add a homologous sequence homologous to a vector as well as a restriction enzyme site (KpnI) to an AtCytb gene. The primer AtC-R of the reverse side is a primer designed to amplify from the start codon of an AtCytb gene to the 726th base. The reason for amplifying until the 726th base is to delete 56 amino acids constituting the flavin domain positioned on the C-terminal side of the AtCytb amino acid sequence in order to cause a subunit having an oxidizing function to be fused. The primer AoC-Kpn-F of the other forward side is a primer designed to add a homologous sequence homologous to a vector as well as a restriction enzyme site (KpnI) to an AoCytb gene. The primer AoC-R of the other reverse side is a primer designed to amplify from the start codon of an AoCytb gene to the 557th base. The reason for amplifying until the 557th base is to delete 5 amino acids positioned on the C-terminal side of the AoCytb amino acid sequence in order to cause a linker sequence to be fused.

Primers: AtC-Kpn-F (SEQ ID NO: 9) 5′-CGAATTCGAGCTCGGGTACCATGCGTTCCTTTCTC-3′ AtC-R (SEQ ID NO: 10) 5′-GTTGGAAACGGTTGCCGGGCACGCT-3′ AoC-Kpn-F (SEQ ID NO: 11) 5′-CGAATTCGAGCTCGGGTACCATGACATTAAGAAAC-3′ AoC-R (SEQ ID NO: 12) 5′-AGGAACTGCATCCTTCGCCAGAGCCCGCCACTTGTCATAGGAG-3′ Reaction conditions: Denaturation at 94° C. for 2 minutes (1 cycle), denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension reaction at 72° C. for 1 minute (30 cycles) extension at 72° C. for 10 minute (1 cycle) 2) Acquisition of a Glucose Dehydrogenase Gene

With the cDNA library of the Aspergillus terreus FERM BP-08578 strain prepared in a manner similar to the methods described in Example 1-1) to Example 1-3) as Template C or the cDNA library of the Aspergillus oryzae NBRC5375 strain as Template D, PCR was performed with the following reaction conditions. As a result of performing PCR using primers AtC-AtG-F and AtG-Kpn-R for Template C, a glucose dehydrogenase (AtGLD) gene fragment E derived from the Aspergillus terreus FERM BP-08578 strain and having a substrate-oxidizing function was acquired. On the other hand, as a result of performing PCR using primers AtG-F and AtG-Kpn-R for Template C, a glucose dehydrogenase (AtGLD) gene fragment F derived from the Aspergillus terreus FERM BP-08578 strain and having a substrate-oxidizing function was acquired. Furthermore, as a result of performing PCR using primers AtC-AoG-F and AoG-Kpn-R for Template D, a glucose dehydrogenase (AoGLD) gene fragment G derived from the Aspergillus oryzae NBRC5375 strain and having a substrate-oxidizing function was acquired. On the other hand, as a result of performing PCR using primers AoG-F and AoG-Kpn-R for Template D, a glucose dehydrogenase (AoGLD) gene fragment I-1 derived from the Aspergillus oryzae NBRC5375 strain and having a substrate-oxidizing function was acquired. Furthermore, the primer AtC-AtG-F of the forward side for amplifying an AtGLD gene is a primer designed to add a homologous sequence homologous to an AtCytb gene to an AtGLD gene and to further amplify from the 73rd base from the start codon of the AtGLD gene. The primer AtG-Kpn-R of the reverse side for amplifying an AtGLD gene is a primer designed to add a homologous sequence homologous to a vector as well as a restriction enzyme site (KpnI) to an AtGLD gene. The reason for amplifying from the 73rd base is that the gene part until the 72nd base is the fluctuation part in the steric structure on the N-terminal side of the amino acid sequence, and therefore, that part is removed. The primer AtG-F of the other forward side for amplifying an AtGLD gene is a primer designed to amplify from the 73rd base from the start codon of an AtGLD gene. The primer AtC-AoG-F of the forward side for amplifying an AoGLD gene is a primer designed to add a homologous sequence homologous to an AtCytb gene to an AoGLD gene and to further amplify from the 76th base from the start codon of the AoGLD gene. The primer AoG-Kpn-R of the reverse side for amplifying an AoGLD gene is a primer designed to add a homologous sequence homologous to a vector as well as a restriction enzyme site (KpnI) to an AoGLD gene. The reason for amplifying from the 76th base is that the gene part until the 75th base is the fluctuation part in the steric structure on the N-terminal side of the amino acid sequence, and therefore, that part is removed. The primer of the other forward side for amplifying an AoGLD gene is a primer designed to amplify from the 76th base from the start codon of an AoGLD gene.

Primers: AtC-AtG-F (SEQ ID NO: 13) 5′-CCGGCAACCGTTTCCAACGCCAAATATGATTATATCGTTATTG-3′ AtG-Kpn-R (SEQ ID NO: 14) 5′-CTACAGATCCCCGGGGTACCCTAACGACGACCAGC-3′ AtG-F (SEQ ID NO: 15) 5′-GCCAAATATGATTATATCGTTATTGGAGGCGGTACTAGCGGTT-3′ AtC-AoG-F (SEQ ID NO: 16) 5′-CCGGCAACCGTTTCCAACACGACATACGACTACATCGTTGTGG-3′ AoG-Kpn-R (SEQ ID NO: 17) 5′-CTACAGATCCCCGGGGTACCCTAAGCACTCTTCGC-3′ AoG-F (SEQ ID NO: 18) 5′-ACGACATACGACTACATCGTTGTGGGAGGCGGCACAAGTGGTC-3′ Reaction conditions: Denaturation at 94° C. for 2 minutes (1 cycle), denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension reaction at 72° C. for 1 minute (30 cycles) extension at 72° C. for 10 minutes (1 cycle) 3) Acquisition of a Linker Sequence Gene

With the cDNA library of the Aspergillus terreus NIH2624 strain prepared in Example 1-3) as a template, PCR was performed with the following reaction conditions. As a result of performing PCR using primers AoC-L-F and L-AtG-R, a linker sequence gene fragment I was acquired. On the other hand, as a result of performing PCR using primers AoC-L-F and L-AoG-R, a linker sequence gene fragment J was acquired. Furthermore, the primer AoC-L-F of the forward side is a primer designed to add a homologous sequence homologous to an AoCytb gene to a linker gene. The primer L-AtG-R of the reverse side is a primer designed to add a homologous sequence homologous to an AtGLD gene to a linker sequence gene. The primer L-AoG-R of the other reverse side is a primer designed to add a homologous sequence homologous to an AoGLD gene to a linker sequence gene.

Primers: L-AoG-R (SEQ ID NO: 19) 5′-GATGTAGTCGTATGTCGTACCGTCAGGGACAGGAACAGGCTCG-3′ AoC-L-F (SEQ ID NO: 20) 5′-GCGAAGGATGCAGTTCCTGGAGACTGCTCCGGCGATGGCGGTG-3′ L-AtG-R (SEQ ID NO: 21) 5′-CAATAACGATATAATCATATTTGGCACCGTCAGGGACAGGAAC-3′ Reaction conditions: Denaturation at 94° C. for 2 minutes (1 cycle), denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension reaction at 72° C. for 1 minute (30 cycles) extension at 72° C. for 10 minutes (1 cycle) 4) Subcloning of a Fusion Body Gene to Escherichia coli

An expression vector pTCTG was created by treating the AtCytb gene fragment acquired in Example 4-1), the AtGLD gene fragment E acquired in Example 4-2), and a vector pUSA with a restriction enzyme KpnI using the In-Fusion (trademark) Advantage PCR Cloning Kit (Manufactured by Takara Bio Inc.). Furthermore, the vector pUSA was subdivided by the National Research Institute of Brewing. Furthermore, an expression vector pOCTG was created by treating the AoCytb gene fragment acquired in Example 4-1), the AtGLD gene fragment F acquired in Example 4-2), and a vector pUSA with a restriction enzyme KpnI using the In-Fusion (trademark) Advantage PCR Cloning Kit (Manufactured by Takara Bio Inc.) along with the linker gene fragment I acquired in Example 4-3). In addition, an expression vector pTCOG was created by treating the AtCytb gene fragment acquired in Example 4-1), the AoGLD gene fragment G acquired by Example 4-2), and a vector pUSA with a restriction enzyme KpnI using the In-Fusion (trademark) Advantage PCR Cloning Kit (Manufactured by Takara Bio Inc.). Additionally, an expression vector pOCOG was created by treating the AoCytb gene fragment acquired in Example 4-1), the AoGLD gene fragment F acquired in Example 4-2), and a vector pUSA with a restriction enzyme KpnI using the In-Fusion (trademark) Advantage PCR Cloning Kit (Manufactured by Takara Bio Inc.) along with the linker gene fragment J acquired in Example 4-3). The expression vectors pTCTG, pOCTG, pTCOG or pOCOG were transformed by introducing each into an Escherichia coli JM109 strain. Using an illustra plasmidPrep Midi Flow Kit (manufactured by GE Healthcare Japan), plasmid was extracted from three clones of each transformant obtained and a sequence analysis of the insert was performed, and consequently, intended genes could be confirmed in all of the plasmids. In this manner, vectors pTCTG, pOCTG, pTCOG, and pOCOG including fusion body genes (SEQ ID NO: 22, 24, 26, and 28) in which (1) an AtCytb gene and an AtGLD gene, (2) an AoCytb gene, a linker sequence gene, and an AtGLD gene, (3) an AtCytb gene and an AoGLD gene, and (4) an AoCytb gene, a linker sequence gene, and an AoGLD gene were respectively fused could be acquired.

EXAMPLE 5 Expression and Purification of a Fusion Body

1) Transformation of a Fungus and Confirmation of Expression of a Fusion Body

Using the pTCTG, pOCTG, pTCOG or pOCOG acquired in Example 4-4), recombinant fungi (Aspergillus oryzae) producing each fusion body were created in accordance with the methods described in the known Documents 1 and 3 (Gene manipulation technology for rice molt fungi for refined sake, GOMI Katsuya, Journal of the Brewing Society of Japan, P494-502, 2000). Furthermore, for the host fungus to be used, as described in the known Document 2 (BioSci. Biotech. Biochem., 61(8), 1367-1369, 1997), those that were bred at a brewing experiment station in 1997 (Heisei 9), utilized for analyses of transcription factors and for breeding of high-producing strains of various enzymes, etc., and subdivided are available. After each transformant was selected in a Czapek-Dox solid medium, each transformant was inoculated in a thick test tube (22 mm×200 mm) in which 10 mL of the liquid medium described in Example 1-1) had been poured, and it was cultured while being shaken at 30° C. for 40 hours. After the culturing was completed, each culture solution was centrifuged (3,000×g, 20 minutes), and those from which the deposition was removed were determined to be crude protein samples. Each crude protein sample was subjected to SDS-PAGE using 15.0% polyacrylamide gel in accordance to the method of LaemmLi, et al. After migration, it was stained with Coomassie brilliant blue (CBB) and the expression of each fusion body was confirmed by comparing the mobility with that of a molecular weight marker (Dyna Marker Protein Recombinant manufactured by Bio Dynamics Laboratory Inc), and consequently, a fusion body AtCytb-AtGLD (SEQ ID NO: 23) in which soluble AtCytb and AtGLD with high substrate specificity had been fused, a fusion body AoCytb-AtGLD (SEQ ID NO: 25) in which soluble AoCytb and AtGLD with high substrate specificity had been cross-linked with a linker sequence and fused, a fusion body AtCytb-AoGLD (SEQ ID NO: 27) in which soluble AtCytb and AoGLD with high substrate specificity had been fused, or a fusion body AoCytb-AoGLD (SEQ ID NO: 29) in which soluble AoCytb and AoGLD with high substrate specificity had been cross-linked with a linker sequence and fused had a band stained at a position of 124 kDa, 115 kDa, 124 kDa or 115 kDa, respectively, and the expression of each fusion body was confirmed. Furthermore, the glucose dehydrogenase activities of the fusion bodies were measured by the enzyme activity measurement method 1 described in International Publication No. 2004/058958, and consequently, the glucose dehydrogenase activities could be confirmed in AtCytb-AtGLD, AoCytb-AtGLD, AtCytb-AoGLD and AoCytb-AoGLD.

2) Seed Culture of a Recombinant Fungus

Two Sakaguchi flasks were prepared as described in Example 1-1) and two types of recombinant fungi created using the pTCTG or pOCTG in Example 5-1) were each inoculated into these Sakaguchi flasks and cultured while being shaken at 30° C. for 62 hours in order to obtain various culture solutions.

3) Actual Culturing

First, 3.5 L of each liquid medium consisting of Pinedex 2% (manufactured by Matsutani Chemical Industry co., Ltd.) (W/V), triptone 1% (manufactured by BD) (W/V), potassium dihydrogen phosphate 0.5% (manufactured by Nacalai Tesque) (W/V), magnesium sulfate heptahydrate 0.05% (W/V) (manufactured by Nacalai Tesque), an antifoam agent, and water was prepared to pH 6.0, poured into each 5-L jar fermentor, and autoclaved at 121° C. for 20 minutes. These liquid media that had been cooled were each inoculated with 45 mL of each culture solution prepared in Example 5-2), and they were cultured at 30° C. for 62 hours under conditions of aeration and agitation. The culture supernatant obtained by filtering each culture solution was used as a crude protein solution. From the crude protein solution, each fusion body was further isolated and purified by the following steps 4)-6).

4) Concentration/Desalination

Pefabloc SC (manufactured by Roche Diagnostics K.K.) was added to each crude protein solution so that each final concentration would be 0.4 mM, was and each solution was concentrated with an ultrafiltration membrane “Pellicon 2 module” (manufactured by Millipore K.K.) with a molecular weight cut-off of 10,000 and substituted into a 50 mM potassium phosphate buffer solution (pH7.0) including 0.4 mM Pefabloc SC in order to prepare a concentrated solution of each crude protein.

5) Purification with Butyl-TOYOPEAL650M (Manufactured by Tosoh Corporation)

Each supernatant obtained by preparing each of the concentrated solutions of the crude protein to a 60% saturated ammonium sulfate solution (pH7.5) and centrifuging each was passed through a Butyl-TOYOPEAL650M column equilibrated with a 50 mM potassium phosphate buffer solution (pH 7.0) including 60% ammonium sulfate in order to cause each intended protein to be absorbed, and after it was washed with the same buffer solution, each protein was eluted with a gradient elution method at an ammonium sulfate concentration of 60% to 30%, respectively. Regarding each intended fraction, fractions having glucose dehydrogenase activity were collected.

6) Purification with DEAE-Cellulofine A-500 (Manufactured by Seikagaku Corporation)

Each of the collected fractions was concentrated with an ultrafiltration membrane “Pellicon 2 module,” and after desalination, they were each equilibrated with a 1 mM potassium phosphate buffer solution (pH 7.0), caused to be absorbed by DEAE-Cellulofine A-500 equilibrated with the buffer solution, and washed with the same buffer solution, and subsequently, each protein was eluted by a gradient elution method with the same buffer solution and a potassium phosphate buffer solution at concentrations of 1 mM to 150 mM in order to collect an intended fraction. Each of the obtained purified proteins was subjected to SDS-PAGE using 15.0% polyacrylamide gel in accordance with the method of LaemmLi, et al. After migration, staining was performed with CBB, and the results of comparing the mobility with that of a molecular weight marker (Dyna Marker Protein Recombinant manufactured by Bio Dynamics Laboratory Inc) confirmed that the respective proteins were single (molecular weight of approximately 124 kDa or 115 kDa). This allowed for the isolation of a fusion body AtCytb-AtGLD in which soluble AtCytb and AtGLD with high substrate specificity had been fused or a fusion body AoCytb-AtGLD in which soluble AoCytb and AtGLD with high substrate specificity had been cross-linked with a linker sequence and fused. In other words, a fusion body in which soluble extracellular secretion type cytochrome having an electron mediator function and glucose dehydrogenase having a substrate-oxidizing function and high substrate specificity had been fused was successfully obtained, isolated, and purified.

Furthermore, the fusion body AtCytb-AtGLD or AoCytb-AtGLD obtained in Example 5-6) had the following physicochemical properties: (1) has absorption spectra (374 nm and 460 nm) characteristic of a flavin adenine dinucleotide and an absorption spectrum (419 nm) characteristic of oxidized extracellular secretion type cytochrome, the peaks at 374 nm, 460 nm, and 419 nm disappeared by adding glucose to a solution including the fusion body, and peaks at 428 nm, 532 nm and 562 nm characteristic of extracellular cytochrome in a reduced form are observed; and (2) has glucose dehydrogenase activity.

EXAMPLE 6

-   Measurement of Electron-Donating Ability from a Fusion Body to an     Electron Mediator

In order to measure electron-donating ability from AoCytb-AtGLD, which is a fusion body in which extracellular secretion type cytochrome had been fused to AtGLD, to cytochrome C, the absorbance (ΔABS) of the cytochrome C at 550 nm was measured in 0.5 mL of a reaction solution consisting of a 50-mM MES buffer solution (pH 6.0), 1.0-mM D-glucose, 0.05-mM cytochrome C, and 1-μM AoCytb-AtGLD or 1-μM AtGLD. A reaction solution excluding 1-μM AoCytb-AtGLD or 1-μM AtGLD was set in a spectrophotometer with an isothermal cell holder, and after incubation at 30° C. for 5 minutes, AoCytb-AtGLD or AtGLD was added so that the final concentration would be 1 μM, and the absorbance (DABS) at 550 nm was measured. The results of the measurement of absorbance are shown in FIG. 3. The absorbance increased together with increases in the reduced cytochrome C, and changes in absorbance indicate the electron-donating ability from the enzyme to the cytochrome C and a steeper slope in the graph indicates a faster reaction speed.

As shown in FIG. 3, the reaction speed of AtGLD decreases in the latter half of the reaction when the concentration of the cytochrome C becomes low, but the reaction speed of AoCytb-AtGLD, which is the fusion body, does not decrease in the latter half of the reaction. Based on this, it is discovered that AoCytb-AtGLD has excellent electron-donating ability for cytochrome C, indicating that it has high affinity with cytochrome C.

EXAMPLE 7

The sensor in the present example was created based on the examples of the sensor described previously. The material of the conductive layer and the electrodes was palladium. After the conductive layer was formed through sputtering deposition on a polyethylene terephthalate substrate, which was an insulating resin, a first electrode (working pole) and a second electrode (counter pole) were formed by providing a nonconductive track on the conductive layer using a YAG laser. A reagent layer was formed by applying a reagent solution in a circle only on the working pole obtained in this manner and drying at room temperature. The composition of the reagent solution included flavin adenine dinucleotide-dependent glucose dehydrogenase derived from Aspergillus terreus (4 units/0.6 microliter), extracellular secretion type cytochrome derived from Aspergillus terreus (equimolar concentration as the enzyme), carboxymethyl cellulose (0.25 weight %), and potassium phosphate buffering agent (15 mM, pH 7.0). Furthermore, the enzyme and the cytochrome were present in the fusion body described in the embodiments. The amount of reagent solution dropped was an amount in which 4 units of the enzyme were contained per sensor. At the same time, on the counter pole, a paste of silver-silver chloride (Ag/AgCl) (manufactured by BAS) was applied and spread over the entire counter pole with a thin rod-like instrument. The applied paste was dried at room temperature. A cavity into which a sample solution was supplied was formed by sticking a spacer having a notch part as well as a cover on this substrate.

The current response of the sensor created as described above was measured using a plurality of glucose solutions with different concentrations. The glucose solutions were created by dissolving glucose in phosphate buffered saline (PBS). The concentration range was set to 0 to 150 mg/dL (8.3 mM), which included blood glucose concentrations slightly higher than normal. For the current response, the glucose solution was introduced into the cavity while applying a voltage of +350 mV to the working pole using a potentiostat (manufactured by BAS, ALS-660B) for the counter pole, and temporal changes of the current after the introduction were measured at 25° C. FIG. 7 shows the concentration dependence of the current value of the introduced glucose solution. The current values plotted in FIG. 7 are the currents at 2 and 5 seconds after the introduction of the solutions. In both currents, increases in the response current value due to increases in glucose concentration (approximately 0.08 to 8.3 mM, 1.56 to 150 mg/dL) were confirmed, and it was possible to measure glucose, which was the subject to be measured, with the present sensor and measuring method. [Industrial Applicability of the Invention]

The electron mediator and the fusion body of the present invention are useful for electrodes for a glucose sensor to measure blood glucose value or electrodes for a battery, etc. The measurement method, electrodes, and sensors of the present invention are useful for measurements of a subject to be measured.

EXPLANATION OF THE SYMBOLS

-   1: Sensor -   2: Substrate -   3: Conductive layer -   31: Working electrode -   32: Counter electrode -   33: Detection electrode -   4: Reagent layer -   5: Spacer -   51: Inlet -   52: Capillary -   6: Cover -   61: Ventilation hole 

What is claimed is:
 1. A sensor for performing measurement of a subject to be measured included in a sample solution, comprising at least: i) an insulating first substrate; ii) first and second electrodes placed on said first substrate; iii) a reagent layer placed on said first electrode; and iv) a sample solution-holding part contacting said first electrode or said reagent layer and said second electrode, wherein said reagent layer includes extracellular secretion type cytochrome and an enzyme is placed on either said reagent layer or said sample solution-holding part; wherein said enzyme is flavin adenine dinucleotide-dependent.
 2. The sensor according to claim 1, wherein either of said first and second electrodes includes any of carbon, gold, platinum or palladium.
 3. The sensor according to claim 1, wherein said second electrode is an oxidizing and reducing electrode.
 4. The sensor according to claim 3, wherein said second electrode is Ag/AgCl.
 5. The sensor according to claim 1, wherein said reagent layer is placed only on said first electrode.
 6. The sensor according to claim 1, wherein said extracellular secretion type cytochrome and said enzyme are fused.
 7. The sensor according to claim 1, wherein the amount of said extracellular secretion type cytochrome is smaller than 100 times that of said enzyme used in measurement.
 8. The sensor according to claim 1, wherein said extracellular secretion type cytochrome is derived from any of a bacterium belonging to filamentous bacteria, or a bacterium belonging to genus Aspergillus.
 9. The sensor according to claim 1, wherein said enzyme is oxidoreductase or dehydrogenase.
 10. The sensor according to claim 1, wherein said enzyme is an enzyme acting on glucose.
 11. The sensor according to claim 1, wherein said enzyme is flavin adenine dinucleotide-dependent glucose dehydrogenase.
 12. The sensor according to claim 1, wherein said subject to be measured is glucose.
 13. The sensor according to claim 1 that further includes a second substrate having a v) notch part, wherein said second substrate forms at least a portion of said sample solution-holding part.
 14. The sensor according to claim 13, wherein the shape of said notch part is rectangular or U-shaped.
 15. The sensor according to claim 13 that further includes a vi) third substrate, wherein said third substrate forms at least a portion of said sample solution-holding part.
 16. The sensor according to claim 1, wherein a measurable concentration of said subject to be measured is greater than 5 mM. 